Trains are delayed between Royston and Cambridge due to a fault with the signalling system.
Services which usually run between King's Cross and Cambridge, Ely and King's Lynn are facing delays as trains have to run at reduced speeds in the area.
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Great Northern Rail (GTR) is advising passengers that their journey may take 30 minutes longer than usual, and they should leave extra time to travel.
Travel on alternative routes will be accepted on Thameslink services at no extra cost.
The disruption is expected to last until 3.30pm.
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