Southern Railway is facing fresh strike threats by workers in a new dispute as it tries to head off a five-day walkout next week in the long-running row over the role of conductors.

Members of the drivers' union Aslef will also vote for industrial action.

Aslef said there had been a breakdown in industrial relations at Southern and Gatwick Express.

The ballot was announced before talks between Southern and the Rail, Maritime and Transport (RMT) union in a separate dispute over the role of conductors.

The RMT is planning five days of strikes from Monday if the deadlocked row is not resolved.

Southern said a "strike timetable" will be put in place if the action is not called off by Thursday afternoon.

Aslef said "ongoing issues" over the imposition of new rosters to facilitate the introduction of an emergency timetable on Southern remain unresolved.

General secretary Mick Whelan was giving Southern and Gatwick Express's owners, Govia Thameslink Railway, statutory notice of the ballot for strike action and other forms of industrial action.

The result is due at the end of the month.

Southern said that by using "contingency conductors" it expects to run almost 60% of its normal timetable, or two-thirds of a temporary, revised timetable it brought in last month to deal with staff shortages, if next week's RMT strikes go ahead.

A statement said: "There will be a restricted service, with many routes having fewer trains, and, unfortunately, on some routes there will be no service at all. Trains on many routes will start late and finish early."

GTR passenger service director Angie Doll said: "We are sorry that our passengers once again look set to suffer further disruption because of the RMT. The union must call this action off by Thursday afternoon if we are to avoid having to put in Monday's strike timetable.

"This action is not necessary. Our new on-board supervisors will no longer close the doors, a task that passes to the driver with the aid of CCTV. This will cost no-one their jobs, and frees up staff on board trains to better serve passengers.

"As many services will have staff on board as they do today and our on-board staff will have an improved role which better meets the needs of passengers, securing their valued position on the railway for the long term.

"And there will be fewer train cancellations in future. Currently, conductors are tied to specific routes and services, but the new on-board supervisor will be able to go anywhere on our network, significantly reducing the perennial problem of train cancellations due to conductors not being available when they're delayed by disruption, for example."

Aslef will be balloting just under 1,100 drivers.

Ms Doll said: "Twice Aslef has tried to block our plans to improve the railway and twice the courts have ruled their attempts unlawful.

"Aslef members have been successfully operating this timetable for three weeks to deliver more reliable services for passengers and staff in the face of train crew shortages. To call a strike ballot against this timetable now is a cynical and desperate attempt to heap even more misery on passengers, especially as we have met with Aslef on two occasions and have a further meeting with them next week on this specific issue.

"Passengers should remember that Aslef and the RMT signed a pact last year to fight any extension of driver-only operation tooth and nail. The news of this ballot comes at the very moment we are trying to find a resolution with the RMT at Acas and is premature and opportunistic."