Commuters in and out of London's Euston station are facing delays of up to 45 minutes until midday as a result of safety checks on the line and a signalling problem in the Milton Keynes area.
According to the National Rail Enquiries website, checks are being made on the line between Hemel Hempstead and Watford Junction which means trains running at slower speeds between Milton Keynes Central and London Euston / Clapham Junction.
There is also a signalling problem near Milton Keynes Central.
Until noon, Virgin Trains will run a reduced service between Manchester Piccadilly / Birmingham New Street and London Euston.
No Southern trains will run between Milton Keynes Central and Clapham Junction and a reduced service will run between Shepherds Bush and Clapham Junction.
London Midland have halted London Euston to Tring services. Alternative services will make additional stops at Apsley and Kings Langley.
A broken down train at Berkhamsted is causing delays of up to 40 minutes between Milton Keynes Central and London Euston.