Trains are expected to be delayed after a Lorry has become trapped under a bridge in St Neots.
The disruption will cause delays between services from Peterborough and Hitchin.
A spokesman for Great Northern Rail said: "Trains are currently required to run at a reduced speed on all lines, and therefore, train services may be cancelled, delayed by up to 20 minutes or revised until the bridge has been inspected and declared safe for trains to travel across at normal speed."
Disruption is expected until 6pm this evening.
Thameslink and Great Northern are accepting each others tickets on all reasonable routes.
To find out more visit the Great Northern website.