Southern Rail workers are to stage a second strike as their summer of discontent over the role of conductors continues. RMT Union members will stage a 48-hour walkout on 7 and 8 September after the rail franchise's owner Govia Thameslink took responsibility for closing carriage doors from conductors and gave them to drivers.

The union said the plans were a "flagrant disregard for the safety and security of passengers". The strike will last between 12.01am on Wednesday 7 September and 11.59pm on Thursday 8 September.

Thousands of commuters' journeys were blighted earlier this month by a five-day strike that was eventually called off on the third day. As well as the industrial action staff have conducted an unofficial strike with soaring sickness.

RMT general secretary Mick Cash said: "This action has been forced on us by the arrogance and inaction of the Govia Thameslink and the government, who have made it clear that they have no interest in resolving this dispute or in tackling the daily chaos on Southern.

"Instead they have begun the process of bulldozing through the drive towards wholesale Driver Only Operation without agreement and without any concern for the impact on safety, security and disability access."

Southern Passenger Service Director Alex Foulds denied there were safety issues and said the action was unnecessary. "Passengers and staff will once again be appalled by the RMT's decision to hold yet another strike," he said. "We are moving forward with our plans for the benefit of customers after nine months of fruitless attempts to reach an agreement.

"This action is unnecessary, unjustified and futile – we have guaranteed all our onboard staff a job until the end of the franchise, as valued members of our future operation, with no reduction in salary. And claims that safety is at risk are just untrue. The independent rail safety body has said so, and nearly half our trains run without conductors already."

Passengers are also likely to be irked at the news that more than 250 Govia Thameslink Railway staff will be balloted in a row over ticket office closures. TSSA union members are against Southern rail chief plans to close 34 ticket offices in a move that would see 47 job losses.