Gatwick Express, Thameslink trains between Brighton and London Bridge, and trains between Gatwick and East Croydon, have all been affected.
Rail operator Southern has said that the problems, caused by signal failure at Purley, are likely to continue until noon today.
For more details and information about alternative routes, visit: http://www.southernrailway.com/your-journey/plan-your-journey/live-running-information/
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