Outsourced workers who clean civil servants’ desks and toilets have begun a four-day strike after missed payments and low pay left them living in poverty and forced them to turn to food banks set up inside the business department.

Baked beans, tins of tuna and potatoes from boxes set up on every floor of the department’s headquarters at One Victoria Street, Westminster – including outside the office of the business secretary, Greg Clark – were circulated among cleaners and security guards in the offices every day last week.

They were among workers from two departments striking this week because of low pay and missed payments.

Workers say they have endured weeks of missed and incomplete wage payments after the contract for their work was taken over by ISS. One security guard who has a decade of experience with the department said he had been transferred multiple times between contractors but had never seen such incompetence.

He said: “How will I feed my son? How will my colleagues feed their families? They promise they will pay and it keeps going and going. How can we live like this? How is a government department allowing this to happen?

“If they can do this inside a government department, imagine what they can do outside government. I hope the minister can see this and be ashamed at what he is putting the workers through.”

On Tuesday morning, ISS cleaners joined catering staff employed by another outsourcer, Aramark, who are also striking for the living wage, on a picket line on Victoria Street. One cleaner said he had seen colleagues in tears on payday. “They have got rent to pay and they can’t tell their landlords that they have to pay later because the billion-pound company that employs them can’t pay out on time,” he said.

Rebecca Long-Bailey, the shadow business secretary, walked two minutes from parliament to address the strikers. She said: “When you have workers … having to use food banks in the one department that’s there to support workers’ rights, we know that something wrong has happened.”

The strikers then marched five minutes across Westminster to the Foreign and Commonwealth Office, where maintenance workers, cleaners and catering staff are employed by Interserve. The company, which recently faced severe financial problems, has restructured holiday pay on the overtime worked by cleaners, and ceased company sick pay that some workers had received for over a decade. An Interserve spokesperson said in the case of sick pay, those workers had been paid company sick pay in error and that the change to statutory sick pay was in line with their contracts.

One Interserve worker at the FCO said the company was withholding two weeks’ wages while it switched to a new payroll period, and had promised it would repay the money to workers when they left the company. Workers who were paid monthly have had to wait six weeks between payments while the company restructures its contract with the FCO.

“I’ve got bills to pay,” he said. “I can’t afford to go six weeks without money. I’ve got to pay six weeks’ mortgage out of two weeks’ money.”

Workers said Interserve was also refusing to recognise their union. “They have told us this is an illegal strike. It’s not. We have followed the rules and balloted properly. We just want to be recognised and we can fight our corner. We’re not asking for special treatment.”

An Interserve spokesperson said: “Interserve successfully secured the FCO facilities management contract last year. This provided a revised operating structure to bring improved local delivery for the services and an overall increase in job numbers.

“Beyond a change to the monthly payment date, there are no plans to make any further changes to our terms and conditions of employment, including any change in the way the company pays for overtime and sick pay. The change in payment dates followed a thorough consultation process and Interserve has taken action to support staff throughout the transition period.”

The business department (BEIS) said: “We value all of our staff at BEIS and recognise that they deserve a fair wage for their hard work, whether directly employed or working for our contractors.

“This is why we have ensured that our contractors align the pay of cleaning, catering, mail room and security staff to the median rates for those occupations as identified in the Annual Survey of Hours and Earnings.”

ISS has been approached for comment.