The two days of Southern strikes scheduled for next week have been cancelled and replaced with one 24-hour walkout on May 20.

The reason for the change is that the dispute over the role of conductors and driver-only trains will be prolonged, according to thehe Rail, Maritime and Transport union (RMT).

After a 24-hour strike last week which disrupted services, RMT members were due to walk out again next Tuesday and Thursday.

A union official said: "We realise this dispute may be prolonged.

"We instruct our members to take strike action by not working any shifts that book on between 00.01am and 11.59pm on Friday, May 20."

The union is embroiled in a fight with Southern over staffing levels.

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RMT general secretary Mick Cash said: "We have reassessed the tactics of the dispute and, recognising the hostile and aggressive stance taken by the company as we fight for the basic principles of rail safety, RMT's executive has decided to switch the next phase of action to May 20.

"Southern fleeces its passengers for a fortune in fares for unreliable and overcrowded services and RMT will not allow safety to be the next thing sacrificed in the dash for fatter and fatter profits on this rail franchise."

A spokesman for Southern said the train operator was pleased that the strike was being postponed, but doubted the RMT's stated reasons for cancelling, citing union members' likely reluctance to strike three times in the same pay period.

He said: "This strike is totally unnecessary. All we are doing is making our staff more accessible and more able to give better customer service to passengers on board our trains, by removing the need for them to close the doors.

"This is how 40% of Southern services operate already.

"No one will be made compulsorily redundant and no one will lose salary."