A fresh row has broken out at Southern Railway over holiday pay as workers prepare for another strike in the bitter dispute over the role of conductors.

The Rail, Maritime and Transport union revealed it had received a letter from the company saying that backdated holiday pay is being withheld from conductors involved in the industrial action until the dispute is over.

RMT general secretary Mick Cash said he was seeking legal advice, sending a message to union members with the headline: "Just When You Think Southern Couldn't Go Any Lower In Attacking You And Making This Personal."

He said: "In all the disputes we have had over the years the RMT has never had a company make it so personal against their own staff.

"Other companies have been hostile and angry at the position the union and its members were taking but at least they treated their staff with dignity. Not Southern. They have declared war on their passengers and staff alike."

The letter from Southern's owners Govia Thameslink Railway (GTR) says: "As the dispute over conductors and DOO (driver-only operation) is ongoing and your members continue to breach their contracts by taking strike action, thereby causing significant loss to the business and disruption to our customers, we will (without prejudice to our rights) withhold payment of any backdated holiday pay from conductors.

"Any conductors who have either worked normally during the dispute, or confirm that they will now work normally during the remainder of the dispute and will not participate in further industrial action, will receive payment.

"Failing that, GTR intends to make the appropriate payment to employees of this grade in the pay run on 30 December 2016, subject to the current dispute being concluded and no further strike dates being called."

The union is staging a 48-hour strike from Friday, with more action planned in the coming weeks.

A Southern spokesman said: "Passengers have had to endure 15 days of strikes so far and seven more days to come. These strikes have caused misery and hardship to people's work and family lives.

"We feel we cannot make these payments to conductors who are currently taking industrial action.

"The fact they have taken strike action makes the calculation of what is owed more complicated. We plan to pay them once the industrial action is over."

Southern said it planned to run 61% of normal services on the strike days.

"We have a further 100 trains running their entire route in driver-only operation and 145 more which run part of their route in driver-only operation.

"In all cases there is still a second member of staff on board the train but the driver is in sole control of the doors.

"We are doing this on the Brighton Mainline, Horsham-Dorking and Reigate-London routes."

Liberal Democrat transport spokeswoman Jenny Randerson said: "When Chris Grayling became Secretary of State he promised to make Southern his priority, but four months on passengers are still facing utter chaos. It's frankly been hell for commuters.

"If this is his idea of a priority, I dread to think what will happen to transport problems further down his to-do list."