Trains from London St Pancras International are being delayed or cancelled after a signalling system fault.
Technical issues at St Albans City means trains running between the major London station and Luton are all severely delayed by up to 45 minutes or ‘revised’.
East Midlands Railway services between Sheffield, Nottingham and Corby to and from London St Pancras have all been affected.
Thameslink services between Bedford, East Croydon, Luton and Rainham have also been disrupted.
Disruption is expected to last until 4pm today.
National Rail said: ‘Trains are being stopped at red signals in each direction, and drivers are being given verbal instructions to continue through an area with signalling problems.
‘This takes extra time and is leading to congestion and service changes.
‘Network Rail response staff are investigating as quickly as possible, and we are monitoring progress updates.’
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