Talks aimed at averting further strikes by Southern Railway conductors have collapsed.

The company and the Rail, Maritime and Transport union blamed each other for failing to break a deadlocked row over the role of conductors and driver-only trains.

Southern services were hit by a 24-hour walkout this week, and two further strikes are now set to go ahead next month at 11am on May 10 and 12.

The union accused Southern's owners, Govia Thameslink Railway (GTR) of issuing a "threatening" letter to staff asking them to sign an undertaking not to take further strike action.

General secretary Mick Cash said: "This is a deliberate attempt to pour fuel on the flames and deflect attention from the core safety issue at the heart of this dispute."

A Southern spokesman said: "We came here for serious constructive talks but sadly the RMT was not prepared to discuss details of the new role.

"We tabled, at their request, high level principles on which to move forward and they have dismissed them out of hand, offering no alternative of their own.

"This is a snub to every commuter on the Southern network who now seem condemned to more days of strikes early next month.

"All we are doing is shifting responsibility for closing a train's doors from the conductor to the driver, so the conductor, in their new role of on-board supervisor, can focus on customer service on the train.

"No-one will be made compulsorily redundant, no-one will get a cut in salary.

"We continue to urge the RMT to review their position and not inflict another totally unnecessary strike on our passengers."

Mr Cash said: "Not only have Southern management today collapsed the talks process but they have also dished out forms to sign to give an undertaking that our members will not take part in any further industrial action during this dispute.

"RMT has issued guidance to members that they are under no obligation to sign this form and recommending that they bin it.

"Southern know that our members are determined to see justice is won in this dispute.

"Southern management also know that the public are on our side on the fundamental issue at the heart of this dispute which is safe staffing of our railways. Our members have been advised to ignore the threats, ignore the bullying and ignore the intimidation."

The union is opposed to moves to switch responsibility for operating train doors from conductors to drivers.

The RMT is also planning to hold an industrial action ballot among station staff at Southern in protest at plans to close a number of ticket offices.