diff --git a/src/mudlib/combat/engine.py b/src/mudlib/combat/engine.py
index 3668b81..8b575cd 100644
--- a/src/mudlib/combat/engine.py
+++ b/src/mudlib/combat/engine.py
@@ -42,6 +42,14 @@ def start_encounter(attacker: Entity, defender: Entity) -> CombatEncounter:
     )
     active_encounters.append(encounter)
 
+    # Cancel any active power-up tasks
+    for entity in (attacker, defender):
+        task = getattr(entity, "_power_task", None)
+        if task is not None:
+            task.cancel()
+            if hasattr(entity, "_power_task"):
+                entity._power_task = None  # type: ignore[attr-defined]
+
     return encounter
 
 
diff --git a/tests/test_power.py b/tests/test_power.py
index 4a521ae..133bf39 100644
--- a/tests/test_power.py
+++ b/tests/test_power.py
@@ -307,3 +307,51 @@ async def test_power_to_number_raises_with_loop(player):
     # Should be increasing toward 80
     assert player.pl > 10.0
     assert player.stamina < 100.0  # deducting stamina
+
+
+@pytest.mark.asyncio
+async def test_combat_start_cancels_power_up(player, nearby_player):
+    """Test starting combat cancels any active power-up task."""
+    from mudlib.combat.engine import start_encounter
+
+    # Start player powering up
+    player.pl = 10.0
+    player.max_pl = 100.0
+    player.stamina = 100.0
+
+    await cmd_power(player, "up")
+    await asyncio.sleep(0.05)  # Let power-up start
+
+    # Verify power-up is active
+    assert player._power_task is not None
+    assert not player._power_task.done()
+
+    # Start combat encounter
+    start_encounter(player, nearby_player)
+
+    # Power-up task should be cancelled
+    assert player._power_task is None or player._power_task.cancelled()
+
+
+@pytest.mark.asyncio
+async def test_combat_start_cancels_defender_power_up(player, nearby_player):
+    """Test starting combat cancels defender's active power-up task."""
+    from mudlib.combat.engine import start_encounter
+
+    # Start nearby_player powering up
+    nearby_player.pl = 10.0
+    nearby_player.max_pl = 100.0
+    nearby_player.stamina = 100.0
+
+    await cmd_power(nearby_player, "up")
+    await asyncio.sleep(0.05)  # Let power-up start
+
+    # Verify power-up is active
+    assert nearby_player._power_task is not None
+    assert not nearby_player._power_task.done()
+
+    # Start combat encounter (nearby_player is defender)
+    start_encounter(player, nearby_player)
+
+    # Power-up task should be cancelled
+    assert nearby_player._power_task is None or nearby_player._power_task.cancelled()