Image caption The operator has said it is ready to hold further talks

Staff on the Caledonian Sleeper service are to strike at the end of September.

The RMT union said working conditions on the overnight trains were causing "intolerable" stress.

It claimed poor staffing levels and insufficient training were among the issues operator Serco had failed to address.

Serco described the planned strike as "completely unnecessary" and said it was ready to hold further meetings with the union.

RMT members working on the sleeper service had previously voted by more than 10 to one to back industrial action, in a ballot with an 80% turnout.

Overtime ban

Their union has now instructed them not to book on shift from midday on Sunday 29 September to 11:59 on Tuesday 1 October.

After the initial strike, RMT members have been told not to work overtime, or on rest days, and not to "act up" in higher-grade roles.

RMT general secretary Mick Cash said: "The union is angry and frustrated that promises made to us to address the serious concerns of the Caledonian Sleeper staff at the end of July have not been honoured and that leaves us with no choice but to activate our industrial action mandate.

"RMT members have been put under intolerable personal stress as a result of the company's mismanagement of the sleeper service.

"They know it, they have promised to do something about it but they have failed to deliver.

He added: "The union remains available for genuine and serious talks."

Caledonian Sleeper upgrade: A 'challenging' introduction

Image caption The en-suite double rooms are a "first" for the Sleeper

At its launch in June the revamped Caledonian Sleeper service was described as a "hotel on wheels".

The £150m fleet consists of 75 carriages with en-suite double rooms.

But the service has been beset by a string of problems.

In June, a technical problem - believed to be the application of emergency brakes - saw passengers stuck at Stafford and forced to complete their journey by bus.

Damage caused to wheels in that incident led to several days of cancellations and delays.

Two weeks later, a rescue train was sent to collect passengers stranded in the Highlands after a service from Fort William broke down.

In August, a Caledonian Sleeper train overshot Edinburgh Waverley station, only coming to a halt after emergency brakes were applied.

The Rail Accident Investigation Branch is investigating the Waverley incident, which is believed to have been caused by a brake pipe valve being left closed when the carriages were split from a Glasgow-bound service at Carstairs.

Serco has admitted there have been problems with the introduction of the new Lowland Sleeper service.

It said the change had been "challenging for everyone involved" and thanked staff for their efforts.

Caledonian Sleeper managing director Ryan Flaherty said the company was "disappointed and frustrated" by the threat of industrial action.

"We have met the RMT and company council on many occasions and the changes they have requested have either been completed or are under way," he said.

These included, he said, the recruitment of more staff at Edinburgh, Glasgow and London for a six-month period whilst the new Mark 5 carriages "settle in".

He added: "This action is completely unnecessary and we urge the RMT to come back and tell us what they want. We are ready to go to mediation and hold further discussions."