diff --git a/app/controllers/concerns/query_params_store_concern.rb b/app/controllers/concerns/query_params_store_concern.rb
new file mode 100644
index 000000000..31c8828ee
--- /dev/null
+++ b/app/controllers/concerns/query_params_store_concern.rb
@@ -0,0 +1,17 @@
+module QueryParamsStoreConcern
+  extend ActiveSupport::Concern
+
+  def store_query_params
+    # Don't override already stored params, because we could do goings and comings with authentication, and
+    # lost previously stored params
+    return if session[:stored_params].present? || request.query_parameters.empty?
+
+    session[:stored_params] = request.query_parameters.to_json
+  end
+
+  def retrieve_and_delete_stored_query_params
+    return {} if session[:stored_params].blank?
+
+    JSON.parse(session.delete(:stored_params))
+  end
+end
diff --git a/app/controllers/users/commencer_controller.rb b/app/controllers/users/commencer_controller.rb
index 8754f24bb..c336738da 100644
--- a/app/controllers/users/commencer_controller.rb
+++ b/app/controllers/users/commencer_controller.rb
@@ -1,11 +1,15 @@
 module Users
   class CommencerController < ApplicationController
+    include QueryParamsStoreConcern
+
     layout 'procedure_context'
 
     def commencer
       @procedure = retrieve_procedure
       return procedure_not_found if @procedure.blank? || @procedure.brouillon?
 
+      store_query_params
+
       @revision = @procedure.published_revision
       render 'commencer/show'
     end
diff --git a/app/controllers/users/dossiers_controller.rb b/app/controllers/users/dossiers_controller.rb
index 6aa397d38..6b1d7bb85 100644
--- a/app/controllers/users/dossiers_controller.rb
+++ b/app/controllers/users/dossiers_controller.rb
@@ -1,6 +1,7 @@
 module Users
   class DossiersController < UserController
     include DossierHelper
+    include QueryParamsStoreConcern
 
     layout 'procedure_context', only: [:identite, :update_identite, :siret, :update_siret]
 
@@ -304,7 +305,7 @@ module Users
       )
       dossier.build_default_individual
       dossier.save!
-      dossier.prefill!(PrefillParams.new(dossier, params).to_a)
+      dossier.prefill!(PrefillParams.new(dossier, retrieve_and_delete_stored_query_params).to_a)
 
       if dossier.procedure.for_individual
         redirect_to identite_dossier_path(dossier)
diff --git a/app/models/prefill_params.rb b/app/models/prefill_params.rb
index 05970e277..8e94828c8 100644
--- a/app/models/prefill_params.rb
+++ b/app/models/prefill_params.rb
@@ -12,7 +12,6 @@ class PrefillParams
 
   def build_prefill_values
     value_by_stable_id = @params
-      .to_unsafe_hash
       .map { |prefixed_typed_id, value| [stable_id_from_typed_id(prefixed_typed_id), value] }
       .filter { |stable_id, value| stable_id.present? && value.present? }
       .to_h
diff --git a/spec/controllers/concerns/query_params_store_concern_spec.rb b/spec/controllers/concerns/query_params_store_concern_spec.rb
new file mode 100644
index 000000000..c2e228944
--- /dev/null
+++ b/spec/controllers/concerns/query_params_store_concern_spec.rb
@@ -0,0 +1,66 @@
+RSpec.describe QueryParamsStoreConcern, type: :controller do
+  class TestController < ActionController::Base
+    include QueryParamsStoreConcern
+  end
+
+  controller TestController do
+  end
+
+  before { allow_any_instance_of(ActionDispatch::Request).to receive(:query_parameters).and_return(params) }
+
+  describe '#store_query_params' do
+    subject(:store_query_params) { controller.store_query_params }
+
+    context 'when params are already stored' do
+      let(:params) { { param1: "param1" } }
+
+      it "does nothing" do
+        session[:stored_params] = "there is alread something in there"
+
+        expect { store_query_params }.not_to change { session[:stored_params] }
+      end
+    end
+
+    context 'when params are empty' do
+      let(:params) { {} }
+
+      it "does nothing" do
+        expect { store_query_params }.not_to change { session[:stored_params] }
+      end
+    end
+
+    context 'when the store is empty and we have params' do
+      let(:params) { { param1: "param1", param2: "param2" } }
+
+      it "stores the query params" do
+        expect { store_query_params }.to change { session[:stored_params] }.from(nil).to(params.to_json)
+      end
+    end
+  end
+
+  describe '#retrieve_and_delete_stored_query_params' do
+    subject(:retrieve_and_delete_stored_query_params) { controller.retrieve_and_delete_stored_query_params }
+
+    context 'when there are no stored params' do
+      let(:params) { {} }
+
+      it 'returns an empty hash' do
+        expect(retrieve_and_delete_stored_query_params).to be_empty
+      end
+    end
+
+    context 'when params are stored' do
+      let(:params) { { param1: "param1", param2: "param2" } }
+
+      before { controller.store_query_params }
+
+      it 'deletes the stored params' do
+        expect { retrieve_and_delete_stored_query_params }.to change { session[:stored_params] }.to(nil)
+      end
+
+      it 'returns the stored params' do
+        expect(retrieve_and_delete_stored_query_params).to match(params.with_indifferent_access)
+      end
+    end
+  end
+end
diff --git a/spec/controllers/users/commencer_controller_spec.rb b/spec/controllers/users/commencer_controller_spec.rb
index 953d2fceb..d64094521 100644
--- a/spec/controllers/users/commencer_controller_spec.rb
+++ b/spec/controllers/users/commencer_controller_spec.rb
@@ -15,6 +15,16 @@ describe Users::CommencerController, type: :controller do
         expect(assigns(:procedure)).to eq published_procedure
         expect(assigns(:revision)).to eq published_procedure.published_revision
       end
+
+      context 'when there are query params' do
+        subject { get :commencer, params: { path: path, any_param: "any param" } }
+
+        it "stores the parameters in session" do
+          subject
+
+          expect(session[:stored_params]).to be_present
+        end
+      end
     end
 
     context 'when the path is for a draft procedure' do
diff --git a/spec/controllers/users/dossiers_controller_spec.rb b/spec/controllers/users/dossiers_controller_spec.rb
index 478792f2d..fe653757a 100644
--- a/spec/controllers/users/dossiers_controller_spec.rb
+++ b/spec/controllers/users/dossiers_controller_spec.rb
@@ -1067,7 +1067,7 @@ describe Users::DossiersController, type: :controller do
             it { is_expected.to redirect_to dossiers_path }
           end
 
-          context 'when prefill values are given' do
+          context 'when prefill values have been stored in session before' do
             let!(:type_de_champ_1) { create(:type_de_champ_text, procedure: procedure) }
             let(:value_1) { "any value" }
 
@@ -1082,6 +1082,10 @@ describe Users::DossiersController, type: :controller do
               }
             }
 
+            before { session[:stored_params] = params.to_json }
+
+            it { expect { subject }.to change { session[:stored_params] }.to(nil) }
+
             it { expect { subject }.to change { Dossier.count }.by(1) }
 
             it "prefills the dossier's champs with the given values" do
diff --git a/spec/models/prefill_params_spec.rb b/spec/models/prefill_params_spec.rb
index 271d5f756..4f3e145a0 100644
--- a/spec/models/prefill_params_spec.rb
+++ b/spec/models/prefill_params_spec.rb
@@ -3,7 +3,7 @@ RSpec.describe PrefillParams do
     let(:procedure) { create(:procedure, :published) }
     let(:dossier) { create(:dossier, :brouillon, procedure: procedure) }
 
-    subject(:prefill_params_array) { described_class.new(dossier, ActionController::Parameters.new(params)).to_a }
+    subject(:prefill_params_array) { described_class.new(dossier, params).to_a }
 
     context "when the stable ids match the TypeDeChamp of the corresponding procedure" do
       let!(:type_de_champ_1) { create(:type_de_champ_text, procedure: procedure) }
diff --git a/spec/system/users/dossier_prefill_spec.rb b/spec/system/users/dossier_prefill_spec.rb
index 0d9657daf..386feeba5 100644
--- a/spec/system/users/dossier_prefill_spec.rb
+++ b/spec/system/users/dossier_prefill_spec.rb
@@ -1,4 +1,5 @@
 describe 'Prefilling a dossier:' do
+  let(:password) { 'my-s3cure-p4ssword' }
   let(:siret) { '41816609600051' }
 
   let(:procedure) { create(:procedure, :published, opendata: true) }
@@ -13,20 +14,16 @@ describe 'Prefilling a dossier:' do
     stub_request(:get, /https:\/\/entreprise.api.gouv.fr\/v2\/etablissements\/#{siret}/)
       .to_return(status: 200, body: File.read('spec/fixtures/files/api_entreprise/etablissements.json'))
 
-    visit new_dossier_path(
-      procedure_id: procedure.id,
+    visit commencer_path(
+      path: procedure.path,
       "champ_#{type_de_champ_text.to_typed_id}" => text_value,
       "champ_#{type_de_champ_phone.to_typed_id}" => phone_value
     )
   end
 
-  context 'when the user already exists' do
-    let(:password) { 'my-s3cure-p4ssword' }
-    let!(:user) { create(:user, password: password) }
-
-    scenario "the user has got a prefilled dossier after signing in" do
-      expect(page).to have_content("Connectez-vous")
-      sign_in_with user.email, password
+  shared_examples "the user has got a prefilled dossier" do
+    scenario "the user has got a prefilled dossier" do
+      click_on "Commencer la démarche"
 
       expect(page).to have_current_path siret_dossier_path(procedure.dossiers.last)
       fill_in 'Numéro SIRET', with: siret
@@ -43,28 +40,41 @@ describe 'Prefilling a dossier:' do
     end
   end
 
-  context 'when this is a new user' do
-    before do
-      allow_any_instance_of(FranceConnectParticulierClient).to receive(:authorization_uri).and_return(france_connect_particulier_callback_path(code: "c0d3"))
-      allow(FranceConnectService).to receive(:retrieve_user_informations_particulier).and_return(build(:france_connect_information))
+  context 'when the user signs in with email and password' do
+    it_behaves_like "the user has got a prefilled dossier" do
+      let!(:user) { create(:user, password: password) }
+
+      before do
+        click_on "J’ai déjà un compte"
+        sign_in_with user.email, password
+      end
     end
+  end
 
-    scenario "the user has got a prefilled dossier after signing up" do
-      expect(page).to have_content("Connectez-vous")
-      page.find('.fr-connect').click
+  context 'when the user signs up with email and password' do
+    it_behaves_like "the user has got a prefilled dossier" do
+      let(:user_email) { generate :user_email }
 
-      expect(page).to have_current_path siret_dossier_path(procedure.dossiers.last)
-      fill_in 'Numéro SIRET', with: siret
-      click_on 'Valider'
+      before do
+        click_on "Créer un compte #{APPLICATION_NAME}"
 
-      expect(page).to have_current_path(etablissement_dossier_path(dossier))
-      expect(page).to have_content('OCTO TECHNOLOGY')
-      click_on 'Continuer avec ces informations'
+        sign_up_with user_email, password
+        expect(page).to have_content "nous avons besoin de vérifier votre adresse #{user_email}"
 
-      expect(page).to have_current_path(brouillon_dossier_path(dossier))
-      expect(page).to have_field(type_de_champ_text.libelle, with: text_value)
-      expect(page).to have_field(type_de_champ_phone.libelle, with: phone_value)
-      expect(page).to have_css('.field_with_errors', text: type_de_champ_phone.libelle)
+        click_confirmation_link_for user_email
+        expect(page).to have_content('Votre compte a bien été confirmé.')
+      end
+    end
+  end
+
+  context 'when the user signs up with FranceConnect' do
+    it_behaves_like "the user has got a prefilled dossier" do
+      before do
+        allow_any_instance_of(FranceConnectParticulierClient).to receive(:authorization_uri).and_return(france_connect_particulier_callback_path(code: "c0d3"))
+        allow(FranceConnectService).to receive(:retrieve_user_informations_particulier).and_return(build(:france_connect_information))
+
+        page.find('.fr-connect').click
+      end
     end
   end
 end