Trains running through Sandy, Biggleswade and Arlesey are facing delays or cancellations due to a signals fault.
And passengers are being advised to allow up to half an hour extra to complete their journey.
The problem has been caused by a fault with the signalling system between Peterborough and Hitchin - meaning the line towards St. Pancras International in London is blocked.
Disruption is expected to continue until around 5pm.