diff --git a/garminexport/cli/backup.py b/garminexport/cli/backup.py
index 72fdeee..66a563e 100644
--- a/garminexport/cli/backup.py
+++ b/garminexport/cli/backup.py
@@ -18,6 +18,10 @@ log = logging.getLogger(__name__)
 DEFAULT_MAX_RETRIES = 7
 """The default maximum number of retries to make when fetching a single activity."""
 
+DEFAULT_USER_AGENT = 'Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/90.0.4430.93 Safari/537.36'
+"""The default `User-Agent` to use for HTTP requests when none is supplied by
+the user.
+"""
 
 def parse_args() -> argparse.Namespace:
     """Parse CLI arguments.
@@ -59,6 +63,9 @@ def parse_args() -> argparse.Namespace:
         help=("The maximum number of retries to make on failed attempts to fetch an activity. "
               "Exponential backoff will be used, meaning that the delay between successive attempts "
               "will double with every retry, starting at one second. DEFAULT: {}").format(DEFAULT_MAX_RETRIES))
+    parser.add_argument(
+        "--user-agent", type=str, default=DEFAULT_USER_AGENT,
+        help="A value to use for the `User-Agent` request header. Use an authentic browser agent string to prevent being blocked by Garmin. A tool such as `user_agent` (`ua`) can be used to generate such values.")
 
     return parser.parse_args()
 
@@ -70,6 +77,7 @@ def main():
     try:
         incremental_backup(username=args.username,
                            password=args.password,
+                           user_agent_fn=lambda:DEFAULT_USER_AGENT,
                            backup_dir=args.backup_dir,
                            export_formats=args.format,
                            ignore_errors=args.ignore_errors,
diff --git a/garminexport/garminclient.py b/garminexport/garminclient.py
index 2ac4674..e221009 100755
--- a/garminexport/garminclient.py
+++ b/garminexport/garminclient.py
@@ -81,18 +81,27 @@ class GarminClient(object):
 
     """
 
-    def __init__(self, username, password):
+    def __init__(self, username, password, user_agent_fn=None):
         """Initialize a :class:`GarminClient` instance.
 
         :param username: Garmin Connect user name or email address.
         :type username: str
         :param password: Garmin Connect account password.
         :type password: str
+        :keyword user_agent_fn: A function that, when called, produces a
+        `User-Agent` string to be used as `User-Agent` for the remainder of the
+        session. If set to None, the default user agent of the http request
+        library is used.
+        :type user_agent_fn: Callable[[], str]
+
         """
         self.username = username
         self.password = password
+        self._user_agent_fn = user_agent_fn
+
         self.session = None
 
+
     def __enter__(self):
         self.connect()
         return self
@@ -118,7 +127,15 @@ class GarminClient(object):
             "embed": "false",
             "_csrf": self._get_csrf_token(),
         }
-        headers = {'origin': 'https://sso.garmin.com'}
+        headers = {
+            'origin': 'https://sso.garmin.com',
+        }
+        if self._user_agent_fn:
+            user_agent = self._user_agent_fn()
+            if not user_agent:
+                raise ValueError("user_agent_fn didn't produce a value")
+            headers['User-Agent'] = user_agent
+
         auth_response = self.session.post(
             SSO_SIGNIN_URL, headers=headers, params=self._auth_params(), data=form_data)
         log.debug("got auth response: %s", auth_response.text)
diff --git a/garminexport/incremental_backup.py b/garminexport/incremental_backup.py
index 7c6b6b3..6c0f883 100644
--- a/garminexport/incremental_backup.py
+++ b/garminexport/incremental_backup.py
@@ -3,7 +3,7 @@ import getpass
 import logging
 import os
 from datetime import timedelta
-from typing import List
+from typing import Callable, List
 
 import garminexport.backup
 from garminexport.backup import supported_export_formats
@@ -15,6 +15,7 @@ log = logging.getLogger(__name__)
 
 def incremental_backup(username: str,
                        password: str = None,
+                       user_agent_fn: Callable[[],str] = None,
                        backup_dir: str = os.path.join(".", "activities"),
                        export_formats: List[str] = None,
                        ignore_errors: bool = False,
@@ -23,6 +24,11 @@ def incremental_backup(username: str,
 
     :param username: Garmin Connect user name
     :param password: Garmin Connect user password. Default: None. If not provided, would be asked interactively.
+    :keyword user_agent_fn: A function that, when called, produces a
+    `User-Agent` string to be used as `User-Agent` for the remainder of the
+    session. If set to None, the default user agent of the http request
+    library is used.
+    :type user_agent_fn: Callable[[], str]
     :param backup_dir: Destination directory for downloaded activities. Default: ./activities/".
     :param export_formats: List of desired output formats (json_summary, json_details, gpx, tcx, fit).
     Default: `None` which means all supported formats will be backed up.
@@ -34,6 +40,7 @@ def incremental_backup(username: str,
     The activities are stored in a local directory on the user's computer.
     The backups are incremental, meaning that only activities that aren't already
     stored in the backup directory will be downloaded.
+
     """
     # if no --format was specified, all formats are to be backed up
     export_formats = export_formats if export_formats else supported_export_formats
@@ -50,7 +57,7 @@ def incremental_backup(username: str,
         delay_strategy=ExponentialBackoffDelayStrategy(initial_delay=timedelta(seconds=1)),
         stop_strategy=MaxRetriesStopStrategy(max_retries))
 
-    with GarminClient(username, password) as client:
+    with GarminClient(username, password, user_agent_fn) as client:
         # get all activity ids and timestamps from Garmin account
         log.info("scanning activities for %s ...", username)
         activities = set(retryer.call(client.list_activities))