Passengers travelling between central London and Gatwick Airport are being hit by delays and cancellations because of a major signalling fault.
Both the Gatwick Express and Southern services to the airport are affected and reduced services are operating.
Southern Rail tweeted: "Train services running between Gatwick Airport and Three Bridges may be subject to delays of up to 20 minutes, alteration and cancellation."
Gatwick Express meanwhile announced that its trains are running only half hourly between London Victoria and Brighton instead of the usual service of up to four trains per hour.
Angry passengers vented their frustration on social media, calling it a "great start for commuters" after the Bank Holiday.
@SouthernRailUK are there issues with trains from east croydon to victoria as now already late due to issues once again with no direct trains from haywards heath to victoria - great start for commuters after bank hols— karen scott (@karensco)April 23, 2019
Network Rail apologised to customers saying engineers had been working through the night to fix the problem but that trains are still having to stop at the affected signal.
— NR South East (@NetworkRailSE)April 23, 2019
Commuters were facing delays elsewhere on London transport on Tuesday morning, too after a person was injured on the railway line.
"There's no service between Arnos Grove and King's Cross while emergency services deal with a casualty on the track," the Piccadilly Line account tweeted.
Service on the Piccadilly Line has now returned to normal.