Image copyright PA Image caption South Western Railway said it was "disappointed" at the vote to strike

Workers on the new South Western Railway franchise have voted to join the industrial dispute over the role of train guards.

The RMT union said the firm had failed to give guarantees on the future of "second, safety-critical" members of staff on all of its trains.

The company said it was "disappointed" at the result and planned to "retain a second person" on its services.

South Western Railway is the fifth rail firm to become involved in the dispute.

RMT union members at Southern, Merseyrail, Arriva Rail North and Greater Anglia walked out at 07:00 BST and plan another strike on Thursday.

'Massive mandate'

RMT general secretary Mick Cash said South Western Railway owner First/MTR had repeatedly been asked to give "cast-iron assurances".

He said: "They have refused to give us those guarantees and it is that failure which left RMT with no option but to declare a formal dispute with the company."

Staff backed walkouts by 4-1 on a turnout of 76%, the union said.

It said the "massive mandate" for strikes and other industrial action would be considered by the RMT executive.

South Western Railway said it planned to increase numbers of drivers and guards and urged the union to "avoid premature strike action".

In a BBC interview in August, franchise boss Andy Mellors confirmed its fleet of new trains would "retain a second person" on board, but refused to say what that person's role would be.

Rail minister Paul Maynard said: "Passengers will face needless delays, even though South Western Railway have been clear no-one will lose their job.

"They plan to keep a second member of staff on board their trains and hire more drivers and guards."

A series of strikes over driver-only trains began on Southern railway in April 2016.