East Midlands Trains (EMT) is warning passengers to expect disruption due to a cracked rail between Luton and London St Pancras.
Some trains have been cancelled and others are subject to delay on EMT's London St Pancras, Nottingham, and Sheffield route and the issues are expected to continue until the end of service today (June 5).
There is a speed restriction over the defective track, which is near West Hampstead, and Network Rail engineers are on site monitoring the issue while repairs are being carried out.
An East Midlands Trains spokeswoman, said: "Some of our trains will be delayed by approximately 15 minutes whilst passing over the defect in the Luton direction only between London St Pancras and Luton.
"Some of our trains have been cancelled to reduce congestion. Currently all xx12 Nottingham to London St Pancras and all xx05 London St Pancras to Nottingham services are cancelled.
"We are however expecting to run the 1904 London St Pancras to Nottingham train.
"Our 1650 London St Pancras to Melton Mowbray will be terminating at Corby and will be shorter than advertised today.
"We do not expect normal working to resume until start of service tomorrow.
"We are sorry for any delays to your journey today."
Ticket acceptance is in place for customers with Thameslink between Bedford and London St Pancras.
Customers for Luton Airport Parkway should change at Luton.
EMT advises that you should continue to travel as originally planned, however, your journey may be delayed by 15-30 minutes.
For more details about journeys to London or the north, and to find out if you may be entitled to compensation (if a delay is over 30 minutes), please visit: https://www.eastmidlandstrains.co.uk/information/media/Disruption-Information/Disruption-on-our-London-St-Pancras-route/