diff --git a/locales/en.json b/locales/en.json
index 2b2f10179..e48ba65ce 100644
--- a/locales/en.json
+++ b/locales/en.json
@@ -487,6 +487,7 @@
     "main_domain_change_failed": "Unable to change the main domain",
     "main_domain_changed": "The main domain has been changed",
     "migration_0021_cleaning_up": "Cleaning up cache and packages not useful anymore...",
+    "migration_0021_venv_regen_failed": "The virtual environment '{venv}' failed to regenerate, you probably need to run the command `yunohost app upgrade --force`",
     "migration_0021_general_warning": "Please note that this migration is a delicate operation. The YunoHost team did its best to review and test it, but the migration might still break parts of the system or its apps.\n\nTherefore, it is recommended to:\n    - Perform a backup of any critical data or app. More info on https://yunohost.org/backup;\n    - Be patient after launching the migration: Depending on your Internet connection and hardware, it might take up to a few hours for everything to upgrade.",
     "migration_0021_main_upgrade": "Starting main upgrade...",
     "migration_0021_modified_files": "Please note that the following files were found to be manually modified and might be overwritten following the upgrade: {manually_modified_files}",
@@ -685,4 +686,4 @@
     "yunohost_installing": "Installing YunoHost...",
     "yunohost_not_installed": "YunoHost is not correctly installed. Please run 'yunohost tools postinstall'",
     "yunohost_postinstall_end_tip": "The post-install completed! To finalize your setup, please consider:\n    - adding a first user through the 'Users' section of the webadmin (or 'yunohost user create <username>' in command-line);\n    - diagnose potential issues through the 'Diagnosis' section of the webadmin (or 'yunohost diagnosis run' in command-line);\n    - reading the 'Finalizing your setup' and 'Getting to know YunoHost' parts in the admin documentation: https://yunohost.org/admindoc."
-}
\ No newline at end of file
+}
diff --git a/src/migrations/0021_migrate_to_bullseye.py b/src/migrations/0021_migrate_to_bullseye.py
index adbaac63d..90757ff34 100644
--- a/src/migrations/0021_migrate_to_bullseye.py
+++ b/src/migrations/0021_migrate_to_bullseye.py
@@ -30,6 +30,44 @@ N_CURRENT_YUNOHOST = 4
 N_NEXT_DEBAN = 11
 N_NEXT_YUNOHOST = 11
 
+VENV_REQUIREMENTS_SUFFIX = ".requirements_backup_for_bullseye_upgrade.txt"
+
+
+def _get_all_venvs(dir, level=0, maxlevel=3):
+    """
+        Returns the list of all python virtual env directories recursively
+
+        Arguments:
+            dir - the directory to scan in
+            maxlevel - the depth of the recursion
+            level - do not edit this, used as an iterator
+    """
+    # Using os functions instead of glob, because glob doesn't support hidden folders, and we need recursion with a fixed depth
+    result = []
+    for file in os.listdir(dir):
+        path = os.path.join(dir, file)
+        if os.path.isdir(path):
+            activatepath = os.path.join(path,"bin", "activate")
+            if os.path.isfile(activatepath):
+                content = read_file(activatepath)
+                if ("VIRTUAL_ENV" in content) and ("PYTHONHOME" in content):
+                    result.append(path)
+                    continue
+            if level < maxlevel:
+                result += _get_all_venvs(path, level=level + 1)
+    return result
+
+
+def _backup_pip_freeze_for_python_app_venvs():
+    """
+        Generate a requirements file for all python virtual env located inside /opt/ and /var/www/
+    """
+
+    venvs = _get_all_venvs("/opt/") + _get_all_venvs("/var/www/")
+    for venv in venvs:
+        # Generate a requirements file from venv
+        os.system(f"{venv}/bin/pip freeze > {venv}{VENV_REQUIREMENTS_SUFFIX}")
+
 
 class MyMigration(Migration):
 
@@ -70,6 +108,12 @@ class MyMigration(Migration):
                 'wget --timeout 900 --quiet "https://packages.sury.org/php/apt.gpg" --output-document=- | gpg --dearmor >"/etc/apt/trusted.gpg.d/extra_php_version.gpg"'
             )
 
+        #
+        # Get requirements of the different venvs from python apps
+        #
+
+        _backup_pip_freeze_for_python_app_venvs()
+
         #
         # Run apt update
         #
@@ -264,6 +308,7 @@ class MyMigration(Migration):
 
         tools_upgrade(target="system", postupgradecmds=postupgradecmds)
 
+
     def debian_major_version(self):
         # The python module "platform" and lsb_release are not reliable because
         # on some setup, they may still return Release=9 even after upgrading to