Security staff at the Department for Work and Pensions will strike for two more weeks as the PCS union escalates its dispute over pay.
Around 200 outsourced workers on the G4S Solutions contract with DWP will walk out for a fortnight starting on 12 August.
The escalation follows a week-on, week-off pattern of strikes that has been running since 17 July, including this week.
The strikes have been running alongside walkouts by the GMB union, which also has members working on the DWP G4S contract.
GMB began industrial action over the same issues in May, causing major disruption as jobcentres were forced to close. Around 1,500 of its members on the contract are carrying out their fourth week-long strike this week.
Last week, GMB accused G4S of having broken minimum-wage and modern-slavery laws over the way it has treated jobcentre security guards.
Workers formally submitted a collective grievance over underpayment of the national winimum wage after claiming they were made to carry out compulsory training on their own time.
In a survey of GMB members on the DWP contract, 96% said they were not paid for the training, bringing their pay below the legal minimum wage. Meanwhile, 91% said they completed the training at home and 62% said they felt pressured to do so.
If security guards do not complete their training, G4S threatens disciplinary action or withholds shifts, according to the union. GMB says that in doing so, the outsourcer has breached the Modern Slavery Act 2015, which makes it illegal to pressure an individual to perform forced or compulsory labour.
As GMB marked its fourth week-long walkout, Eamon O’Hearn, GMB's national officer, said: “Not content with paying hard working security guards just the minimum wage, G4S is pressuring them to do unpaid training in their own time.
“GMB believes this constitutes a breach of national minimum wage laws and the Modern Slavery Act.
“G4S gets millions in taxpayer cash to run the job centre security contract for DWP.
“Instead of forking out for expensive agency staff and lining director and shareholder pockets, that money should be used to make sure these workers can live.”
PCS has accused G4S of bringing in workers from agencies or its other contracts – potentially unlawfully – to dampen the effect of the strikes.
PCS general secretary Fran Heathcote said: “Our members risk their own safety to keep DWP staff and the public safe. They deserve to be paid a decent wage. It is wrong that G4S refuse to pay this, particularly while taxpayers are funding their profits. They are a huge multi-national company with substantial assets yet they are paying the minimum wage to their workers.
“It’s time they gave our members the pay and terms and conditions they deserve.”
CSW has asked G4S for a comment.