Workers on South Western Railway are to strike for 27 days before the end of the year in the long-running dispute over guards on trains.

Members of the Rail, Maritime and Transport union will walk out for 27 days between December 2 and New Year’s Day, threatening travel chaos for commuters and Christmas shoppers.

RMT general secretary Mick Cash said: “Our members have been left with no choice but to call a further 27 days of strike action on South Western Railway.

“At the last meeting we held with SWR, principles in agreements were made in good faith with the company’s negotiating team and we now feel hugely let down again.

“As long as the company continues to refuse to give assurances on the future operational role of the guard, we will remain in dispute.

“I want to congratulate our members on their continued resolve in their fight for safety and the role of the guard on SWR.

“It is wholly down to the management side that the core issue of the safety critical competencies and the role of the guard has not been agreed.”

RMT members will not book on for duty from 00:01 on Monday December 2 until 23:59 on Wednesday December 11, from 00:01 on Friday December 13 until 23:59 on Tuesday December 24, and from 00:01 on Friday December 27 until 23:59 on January 1.

The RMT accused the company of “dangling” a potential breakthrough deal in front of the union and then failing to honour it, offering no reasons for the delay.

Prime Minister Boris Johnson’s official spokesman said: “The threat of deeply damaging Christmas strikes by the RMT on South Western Railway is unacceptable.

“Every train which does not run as a result of these unnecessary strikes impacts thousands of passengers at a very busy time when families and friends want to come together.

“We urge the RMT and South Western to get round the table and remove the threat of this damaging action.”

Anthony Smith, chief executive of the independent watchdog Transport Focus, said: “Yet more strikes are being dumped on passengers who may have to cancel Christmas holiday plans or endure miserable journeys to work. This dispute has dragged on for far too long and is damaging trust in the railway.

“Passengers have had enough of the on-going industrial action on South Western Railway services. They have faced lost time, money, more driving and deep frustration at not being able to rely on the trains.

“It is vital that the parties in this dispute get back around the table to resolve this matter without bringing the railway to a standstill.”

A South Western Railway spokesman said: “We are extremely disappointed that the RMT union has once again called for their members to take disruptive industrial action.

“The deliberate targeting of services up to, and during, the Christmas period is typical of the lack of concern the RMT continue to have for our customers.

“The RMT has always said it wanted us to keep the guard on every train. That is what we have offered as part of a framework agreement if the RMT work with us to agree a method of operation for our new trains which fully utilises the new technology to improve safety, security and accessibility as well as day-to-day performance.

“Instead, the RMT appear purely focused on keeping control of train doors in a misguided attempt to hold power over the industry.

“Whilst we have shown commitment to the role of the guard by introducing over 80 additional guard roles since the start of our franchise, the RMT do not have the long-term interests of either our customers or our colleagues, including their members, at the heart of their actions.

“We remain committed to finding a solution that will help us build a better railway for everyone. We will do everything we can to keep customers moving during strike action.”