A first wave of strikes by outsourced security staff forced the Department for Work and Pensions to close dozens of jobcentres to the public this week, the GMB union has said.
More than 1,000 security officers working for G4S Solutions on its contract with DWP walked out over pay on Tuesday and Wednesday. One-day strikes are planned next week and the following week, with another two-day strike at the end of the month.
CSW understands that around 70 jobcentres were unable to provide business-as-usual services as a result of the industrial action, roughly 10% of the total. The strike also impacted administrative centres.
The GMB said support for the walkout was particularly strong on Merseyside and at jobcentres in outer London.
It also accused DWP and G4S of drafting in untrained agency staff to provide cover for striking members, which it said put departmental staff and service users at risk.
The strike centres on pay for guards on the DWP contract, who currently get the National Living Wage of £11.44 an hour if they are aged 21 and over. Reinstatement of pay differentials between grades is also an issue.
GMB national officer Eamon O'Hearn said G4S is refusing to talk to the union. He added that the union had evidence that there was an agreement between G4S and DWP on the use of agency staff during the dispute.
O'Hearn said the walk-out was "historic" and the first time G4S security guards had ever taken strike action over pay.
"They're standing strong: there's overwhelming support from across the country for closures," he said.
"They've had enough and they're finding their voice and their strength and they're being supported with overwhelming numbers of letters and messages from across the world.
"DWP now has a problem and we're calling on it to join us and G4S in resolving this. They can't stand back any more and pretend it's not their problem.
"They should be listening to more than G4S in this dispute and we welcome the opportunity to resume talks with DWP at the table too."
O'Hearn said the use of agency staff for security was "highly contentious" and that the union had "severe concerns" about the implications for safety.
He said he had personally witnessed the consequences from a picket line this week, where police had to be called out because someone who would not normally have been allowed into a jobcentre gained access.
CSW sought responses from G4S and DWP about the scale of jobcentre closures and the suggestion that untrained agency staff had been deployed as a result of the industrial action.
A G4S spokesperson said: "We're disappointed that the GMB has refused to take our improved pay offer to their members. We are continuing to try to reach an amicable agreement, and have implemented contingency plans to minimise disruption to our customer."
They added: "G4S is complying with the law regarding agency staff during the ongoing industrial action."
A DWP spokesperson said: "DWP payments are not affected by the strike action by external G4S staff who work in our jobcentres and most sites remain open. If a site is closed, we are contacting customers if appointments need to be rescheduled or moved to alternative locations."
Members of the PCS union who work for G4S on the DWP contract are currently being balloted on strike action over pay.