Commuters are facing “major disruption” – including cancellations and delays – caused by a signalling fault.
A problem on the line between Luton and Harpenden has left northbound and southbound lines through the area blocked, Network Rail said.
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It said it is expecting the Thameslink service between Brighton and Bedford to be disrupted until at least 13:00 BST.
Services between London St Pancras and Nottingham, Leicester, Derby and Sheffield are also affected.
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Roger Perkins from Thameslink said the problem started just before 02:00 BST.
“Network Rail have been doing their best to try to get to the bottom of what it is,” he said.
“They’ve been steadily restoring bits of the signalling but we’re still in a situation [where] we can’t run anything and that has a very major effect on the services we run, or in fact can’t run unfortunately,” he said.
“It means that passengers trying to get into work today are really going to face a lot of disruption.”
National Rail has warned passengers services may be cancelled or delayed by up to 90 minutes.
Tickets are being 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.
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