Cleaners at Waterloo station in dispute with a private contractor over victimisation will strike again on Friday, responding to their employer’s use of scab labour.

Workers employed by Interserve will down mops for the third time in less than two weeks in protest against victimisation and poor employment practice.

Rail union RMT said Interserve had drafted in agency workers from across London to run a skeleton sweep-up of the station when staff walked out last week. The union has warned Interserve that such action amounts to a breach of the law.

But an Interserve spokesman suggested they would again draft in scabs, speaking of “contingency plans in place to minimise any disruption to services.”

“We have been working with the union to investigate the issues raised,” the spokesman said. “We are committed to providing a safe and supportive environment for all our staff and take allegations of this nature extremely seriously.”

Workers point to the underpayment of wages, racism, bullying and cases of unpaid dismissal among a bucket of grievances they are seeking to resolve.

RMT general secretary Mick Cash said: “RMT and the entire trade union movement salutes the rock-solid strike action of the Interserve workers last month and their determination to secure workplace justice.

“Due to the intransigence of the company we now have no option but to declare a third wave of action.

“As travellers are heading home on Friday full of festive bonhomie they should stop and talk to the station cleaners about their shocking working conditions.”

Workers will walk out from 3pm on Friday and maintain a picket line throughout the evening rush hour.