Govia Thameslink Railway (GTR), the largest rail operator in the UK, has issued a "do not travel" alert following a train derailment and major signaling problems across southeast England.

Southern, Thameslink, and Gatwick Express urged passengers to avoid traveling "if at all possible." The disruption began after a train derailed inside a maintenance depot in Selhurst, followed by a signaling failure on the tracks between London Blackfriars and Norwood Junction.

National Rail Enquiries explained that the signaling fault caused a bottleneck between Norwood Junction and London Blackfriars, forcing trains to move at slower speeds. As a result, several routes were canceled or changed, including services between London Bridge and East Grinstead, Orpington and Luton, Brighton and Cambridge, and Bedford and Three Bridges.

GTR accounts for roughly 18% of all train journeys in the UK. The disruption comes as Great Britain moves to bring its rail operators back under government control. West Midlands Trains was nationalized just last weekend, and GTR is expected to follow suit this May.