diff --git a/src/borg/helpers.py b/src/borg/helpers.py
index df2a136fd..20b0d1341 100644
--- a/src/borg/helpers.py
+++ b/src/borg/helpers.py
@@ -200,6 +200,7 @@ class Manifest:
         self.repository = repository
         self.item_keys = frozenset(item_keys) if item_keys is not None else ITEM_KEYS
         self.tam_verified = False
+        self.timestamp = None
 
     @property
     def id_str(self):
@@ -245,7 +246,13 @@ class Manifest:
         from .item import ManifestItem
         if self.key.tam_required:
             self.config[b'tam_required'] = True
-        self.timestamp = datetime.utcnow().isoformat()
+        # self.timestamp needs to be strictly monotonically increasing. Clocks often are not set correctly
+        if self.timestamp is None:
+            self.timestamp = datetime.utcnow().isoformat()
+        else:
+            prev_ts = datetime.strptime(self.timestamp, "%Y-%m-%dT%H:%M:%S.%f")
+            incremented = (prev_ts + timedelta(microseconds=1)).isoformat()
+            self.timestamp = max(incremented, datetime.utcnow().isoformat())
         manifest = ManifestItem(
             version=1,
             archives=StableDict(self.archives.get_raw_dict()),