Rail passengers are being warned of "major disruption" after a fault in the signalling system in Bedfordshire.
National Rail has said it is expecting the Thameslink service between Brighton and Bedford to be disrupted until at least 1pm.
Tickets will be accepted on Great Northern, South West Trains and Southern services via any reasonable route, buses between Elephant and Castle and Elstree and Borehamwood, and the London Underground by any reasonable route.
Services running between London St Pancras and Nottingham, Leicester and Derby in the East Midlands, as well Sheffield, will also be disrupted.
National Rail has warned passengers services may be cancelled or delayed by up to 90 minutes.
Network Rail is investigating the fault which is on the line between Luton and Harpenden and has resulted in north and southbound lines being blocked.
This story is being updated