Conductors on Southern Railway are to stage three 24-hour strikes in a row over their role and driver-only trains.

Members of the Rail, Maritime and Transport union will walk out from 11am on April 26 and from the same time on May 10 and 12.

The move follows a huge vote of 306 in favour of strikes, with 14 against, and by 320 to one for other forms of industrial action.

The union is opposed to plans to change the role of conductors on Southern, which runs busy commuter services into London from across the South of England.

Parent operating company Govia Thameslink Railway (GTR) wants drivers to open and close train doors under changes it says will make employees more visible and of greater help to passengers.

RMT general secretary Mick Cash said: "RMT, supported by our Aslef colleagues, is wholly opposed to the attack on the safety-critical conductor grades and the threat of driver-only operation.

"Our members on Southern have given loyal and professional service to the company and are being rewarded by being forced into a new grade, having their role and responsibility reduced and their hard-earned terms and conditions attacked.

"The anger at the threat to RMT members' jobs, their role and the safety of Southern services is reflected in this massive vote for action.

"This company has already axed catering services, threatened ticket offices and delivered appalling levels of customer service in their drive to milk these routes for every penny they can regardless of the impact on safety, reliability and quality.

"These trains are desperately over-crowded and the conductors are the eyes and ears preventing a major tragedy on the platforms and carriages."

GTR says giving drivers responsibility for operating the doors would give more flexibility to guards and conductors.

It has promised no compulsory job losses or cuts in pay, adding that some staff would be better off because they could switch to revenue protection work.

The current conductor role will remain on some older trains, but is planned to change on newer rolling stock being introduced in the coming weeks.

GTR is facing a series of disputes with unions threatening industrial action across its network, which includes Southern, Great Northern, Gatwick Express and ThamesLink.

The company is to take legal action against Aslef over its members' refusal to drive new, longer trains on the Gatwick Express.

The union says it has never reached an agreement for the new trains on the Gatwick Airport to London Victoria service.

The company said Aslef members had refused to drive the first two services that had been due to operate in 12-car formation over the past 10 days.

Aslef is planning to ballot its members for industrial action over the dispute.

The union maintains that longer trains operated only by drivers are unsafe. It believes passengers want guards on trains.

Officials say the Department for Transport is behind the drive for more driver-operated services.

Unions are also protesting at plans to close ticket offices.

A spokesman for Southern Rail said: "A strike would be unnecessary and damaging. The changes we are making to the conductor role mean there will be no job losses and no reduction in salary for any staff, whilst passengers will benefit from having more visible staff on trains.

"We are preparing contingency plans in case a strike does go ahead but in the meantime we urge the RMT to return to the negotiating table and discuss the changes to the conductor role we are seeking to make."

Govia Thameslink warned passengers that the strikes will cause "significant" disruption to Southern services.

Dyan Crowther, Chief Operating Officer for GTR, said: "This strike is completely unnecessary. No staff will lose their jobs or see a reduction in their salary, whilst passengers will benefit from having more visible staff on trains.

"Drivers have operated the doors on many trains for 25 years, and already do so on 60% of our services. This is a proven, tried and tested way of operating rail services.

"Now that strike times and dates have been announced, we are working to finalise contingency plans for our services on strike days, but must warn passengers that disruption is likely to be significant.

"Finally, rather than making passengers suffer through a strike, we urge the RMT to return to the negotiating table and discuss the changes we are seeking to make."

Transport Secretary Patrick McLoughlin said: "I condemn any industrial action that will disrupt the travelling public. I expect to see operators and the unions working together to resolve these issues.

"Rail passengers will not thank the unions for inflicting this unnecessary disruption. It is clear that the changes GTR are proposing will modernise services and provide better journeys for passengers."