A fault with the signalling system at Heathrow Airport is disrupting services running between Terminals 2 and 3 and Terminal 5.
The problem is forcing Heathrow Express to run a reduced service throughout the day. Two trains are departing from Paddington every hour instead of the usual four, with a similar reduction in service in the other direction from Heathrow.
Heathrow Express is flagging on its website that the reduced service is in place for the rest of Monday.
The issue is also affecting Elizabeth line trains with no service between Terminals 2 and 3 and Terminal 5.