Commuters are urged to use alternative routes or allow longer for their journey this morning, following an incident between West Hampstead and St Pancras International.
The incident has caused disruption to all trains into London from Bedford.
Trains between Bedford and London St Pancras may continue to be delayed by up to 40 minutes, cancelled or revised while the service returns to normal.
The disruption is expected to continue until 11am.
A spokesman for Thameslink said, “Please consider an alternative route for your journey, and allow. significant amount of extra time.
“We have arranged for tickets for to be accepted on alternative routes but we strongly advise passengers to check their journey using the National Rail Enquiries real-time journey planner.
“If you have started to travel on an alternative route, then your ticket will continue to be accepted whiles you compete your journey.”