Two 24-hour strikes next week by Southern Railway conductors have been cancelled and replaced with a 24-hour walkout on May 20.

The Rail, Maritime and Transport union said the decision was taken because the dispute over the role of conductors and driver-only trains will be prolonged.

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

A union official said: "We realise that this dispute may be prolonged. We therefore cancel the strike action called for Tuesday 10th May to Wednesday 11th May and for Thursday12th May to Friday 13th May.

"Instead, we instruct our members to take strike action by not working any shifts that book on between 0001 and 2359 on Friday 20th May."

The RMT said it will be taking legal advice on the "threatening and abusive" stance the company is adopting towards union members.

The union added it was preparing a fresh wave of campaigning, targeted at building public support for its fight with Southern over staffing levels.

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 should be under no illusions - the union will not bend to their bullying and threats, and we will be building on the huge public support we have already mobilised in a campaign that pitches the demands for increasing private profits against safety on our railways.

"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: "We are naturally pleased that next week's strike has been postponed, but remain concerned that the threat of further strike action hangs over our passengers with a new date announced for May 20.

"While it would be good to think the RMT has changed its position, we believe they have taken this action after members rebelled at the prospect of losing a significant sum in pay and benefits due to the union calling three strike days in the same pay period.

"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.

"We hope this additional time will offer the RMT the opportunity to pause and reflect on the impact their action is having on people and engage in meaningful discussions with us."