Commuters were held up when their homeward-bound train stopped for almost ten minutes to let passengers go to the toilet.

Travellers on the 18.07 from London Victoria had to wait at Haywards Heath for the unscheduled rest stop.

But the break came as a welcome relief for many passengers as they were allowed off to use the station loos after finding the train toilets locked.

The electronic doors of the train loos had locked automatically because the tanks were full and had not been emptied before leaving Victoria.

New regulations mean waste can no longer be drained on to the track while the train is in motion.

The tanks can now only be emptied at the end of a journey.

There have been difficulties with Southern's £800 million Electrostar fleet since it was introduced in Sussex at the end of 2002.

Many passengers have complained about toilets locking automatically.

A spokesman for Southern said: "The train stopped for nine minutes at Haywards Heath to let passengers use the toilets on the platform.

"This was because the tanks were full and we weren't able to de-tank before it left the depot."

He said it was a choice of not putting on the service or putting it on and having a "needs break".

He said: "People on the train were saying thank you to the conductor for being considerate.

"We have to consider our passengers' comfort and we recognise the need for a toilet break between Victoria and Brighton because a lot of people on that particular service would be having drinks from the trolley."

Waste is stored on board and needs to be emptied by special equipment.

The equipment only exists at certain depots, including Brighton, and more need to be constructed.

The incident on Monday is the latest in a series of problems which have beset the Electrostars, which are replacing the old slam-door trains.

However the Southern spokesman said: "It is only the second time this has happened in the past three or four months."

Locked doors on the toilets have caused problems for many passengers during the year.

In May, The Argus reported how three-year-old Bethan Crotty was left distraught by a 40-minute journey on a train with no working toilets.

Bethan was forced to relieve herself in a carriage.

There have been many other problems since the fleet was introduced to replace slam-door carriages.

In a separate incident, more than 1,000 commuters were stuck in carriages for three hours after a power failure.