Thameslink services between St Albans City station, Sutton and Wimbledon have been suspended.
The disruption began at approximately 10.33am this morning (Tuesday, August 30) due to a fault with the signalling system in the Farringdon and City Thameslink areas of central London.
As a result, trains were having to run at reduced speeds through the "core" section of the network.
To ease congestion, the St Albans to Sutton and Wimbledon service - through Cricklewood, Kentish Town and London St Pancras International - was halted.
The first train to run between Sutton to St Albans, via Wimbledon, following the disruption was the 1.49pm departure.
There were also fewer trains than usual between Bedfordshire, Hertfordshire, London and Gatwick Airport into this afternoon.
Passengers on trains running between Farringdon and City Thameslink experienced delays of between 20 and 25 minutes.
A spokesperson for Thameslink said: "Services between St Albans and Sutton/Wimbledon are suspended.
"You'll need to use an alternative route to complete your journey.
"For help planning this, you can use our alternative routes maps."
Alternative route maps are on the Thameslink website (https://www.thameslinkrailway.com/).
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