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More than 600,000 Tube commuters face travel chaos with train drivers on two lines set to walk out in a dispute dubbed the toilet break strike.

Workers on the Hammersmith & City and Circle lines are due to start a 24-hour stoppage from 9pm tonight, claiming their toilet breaks are being timed by stopwatch.

There will be no trains running on the Hammersmith & City line, while the Circle line will have far fewer trains than usual.

A spokesman for the RMT union, which ordered the strike, said: “The dispute is over heavy handed and aggressive management, including putting staff toilet breaks on a stopwatch and flagrant disregard for agreed policies and procedures.”

The stoppage comes after managers reportedly noticed a minority of drivers were spending so long in the toilets - allegedly to avoid work - that their train has had to be cancelled or a new driver found.

Only around 50 of 260 drivers are believed to have voted for the strike action, with the dispute said to be centered on the Edgware Rd and Hammersmith depots.

Peace talks to try and avert the strike broke down late yesterday at Acas, the conciliation service.

Behind the scenes efforts were being made today to try and get the walkout called off but there were no scheduled talks planned.

Services on other Tube lines will not be affected but are also expected to be far busier.

Transport for London (TfL), which is putting on extra buses, warned that some roads in west and central London could also carry more traffic that usual as commuters use alternative routes.

Passengers face more problems all weekend with the two lines closed between Hammersmith and Edgware Road due to engineering work.

Train drivers belonging to the rival union Aslef are not involved but some could refuse to cross RMT picket lines.

Steve White, operations director for London Underground, said: “If the strike goes ahead customers are advised to check our real-time travel information.

“All other Tube lines and TfL services will be operating as normal and we’ll be making sure that everyone has the information they need to re-plan their journeys is necessary.”

Of the “stopwatch” claim, a TfL spokesperson said: “We would refute that. Our staff are able to take reasonable toilet breaks.”

RMT General Secretary Mick Cash said: “This dispute is about the basic issues of protecting working conditions of our members and defending agreements from attempts to drive a coach and horses through them.”