A fresh row has flared between the biggest rail union and Southern Railway ahead of another strike in the bitter dispute over the role of conductors.

The Rail, Maritime and Transport union accused the company of "bullying and intimidation" after a letter was sent to workers urging them not to strike.

RMT members will walk out for 24 hours on Wednesday- the 29th strike since the dispute started almost a year ago.

Angie Doll, Southern's passenger services director, started the letter: "I am sorry to have to write to you again in these terms. I had hoped that we would have put the industrial action behind us."

She said the strikes had caused "immeasurable damage" to passengers, staff, the regional economy and the rail industry.

"We must bring this dispute to an end for everyone's sake," she wrote, adding that services had improved in recent weeks.

"It would be a great shame if this progress was lost due to further industrial action.

"If you do participate in the industrial action you will be unpaid for any shift that you are rostered to work.

"I would like to remind you that taking part in strike action will put you in breach of contract.

"Furthermore we will consider applying additional individual sanctions should RMT call further action in which you participate."

RMT leader Mick Cash said: "Instead of getting back into serious talks Southern Rail have gone back to a campaign of threats, bullying and intimidation of their staff who are doing nothing more than standing up for passenger safety. It is disgraceful behaviour.

"RMT's action remains on and we will not bend to these kind of disgusting dirty tricks from a company that puts private profit before public safety."

A spokesman for Southern's owners, Govia Thameslink Railway, said: "This is a letter from employer to employee urging them to stop this pointless strike and to stop the misery and disruption their action is having on the hardworking travelling public and the regional economy".

Southern has warned that the strike will cause disruption to services. The company remains in dispute with the drivers' union Aslef after its members rejected a deal aimed at resolving a row about driver only trains.