The legal net has started to tighten around the operator of the Fukushima Daiichi nuclear power plant, as victims of the accident, and those responsible for clearing it up, take their grievances to the courts.

Last week, Tokyo Electric Power (Tepco) said it would not contend a court ruling ordering it to pay almost $500,000 in compensation to the family of a woman who killed herself two months after being forced to flee her home near the plant.

That claim, which could pave the way for similar suits, has been followed by a unprecedented attempt by four Fukushima Daiichi workers to sue the utility for unpaid wages.

The two former and two current workers, aged in their 30s to 60s, claim that Tepco and its contractors failed to ensure they were paid mandatory hazard allowances, on top of their regular wages.

In the first legal challenge of its kind against Tepco, the four men, who are not named and wore masks in court for fear of reprisals from their employers, are seeking $600,000 in unpaid wages from Tepco and several of its partner firms.

The men's lawyer said he believed more could follow among the 6,000 workers – most of whom work for contractors – involved in the dangerous 40-year operation to decommission the plant.

"A year ago, the prime minister told the world that Fukushima was under control. But that's not the case," Tsuguo Hirota told Reuters. "Workers are not getting promised hazard pay and skilled workers are leaving. It's becoming a place for amateurs only, and that has to worry anyone who lives near the plant."

The hazardous nature of work to control the flow of radioactive water, and to prepare damaged reactors for the removal of melted nuclear fuel prompted Tepco to announce late last year that it would double daily danger money payments to $200 per worker.

But labourers employed by some of the 800 firms involved in decommissioning Fukushima Daiichi say the extra cash is being withheld by contractors and subcontractors, which claim they need the extra cash to keep their businesses afloat.

"My health could suffer one day … I believe there are many people who can't speak out about this kind of problem," one of the workers told public broadcaster NHK. "I may get fired or may be given no more work. But I hope people will take this as an opportunity to speak up and get paid."

The suits have opened a new front in the legal battle against Tepco, which is expected to pay more than $48bn in compensation to residents affected by the March 2011 disaster, and billions more on decontamination and decommissioning.

Last month, a citizens' judicial panel ruled that three former Tepco executives should face criminal charges over the disaster. Prosecutors must respond to the panel by next month.