Hundreds of inexperienced contractors at the Fukushima Daiichi nuclear plant are working in searing heat and high radiation levels for as little as 10,000 yen (£80) a day.

As Japan succumbs to a heatwave, the plant's operator, Tokyo Electric Power (Tepco), has moved to address rising concerns over the health of almost 3,000 workers battling to bring its stricken reactors under control.

So far, about 9,000 workers have been involved in the four-month operation to stabilise the Fukushima nuclear plant, where three of six reactors suffered meltdown after it was struck by a 15-metre (49ft) tsunami on 11 March.

Nine people, all employees of Tepco, have been exposed to radiation levels exceeding the maximum annual limit of 250 millisieverts. But the longer the crisis drags on, the more the firm will have to depend on hundreds of small subcontractors who hire inexperienced labourers to perform the dirtiest jobs on one of the most hazardous industrial sites on earth.

The Guardian spoke to several construction workers, who said they were paid about 12,000 yen a day to clear radioactive debris left in the tsunami's wake. By contrast, Tepco employees earn an average of 7.6m yen a year.

Testimony from workers suggests that already hazardous conditions inside the plant have worsened by the arrival of a hot spell that shows no sign of relenting.

The first few months of the Fukushima operation exposed several lapses in health and safety. Some men who were on site in the early days of the crisis were not screened for internal radiation exposure until June, while others had to share dosimeters, which measure radiation levels.

In May, a 60-year-old man who had been working at the No 4 reactor died. The official cause of death was heart failure, possibly brought on by working successive 12-hour shifts. Dressed in protective boiler suits, gloves and masks fitted with filters, more than 2,500 contract workers endure searing heat during shifts that last no more than 90 minutes at a time.

"We are very concerned about the health of all the workers on site and have taken appropriate measures to protect them against the heat," said Yoshikazu Nagai, a Tepco spokesman.

Of the 2,902 people working at the site, only 373 are Tepco employees. The remainder perform myriad tasks, from removing rubble and debris to monitoring radiation levels and decontaminating uniforms and equipment.

In response to mounting criticism of conditions at Fukushima, the utility says it has increased the number of rest areas, installed more water coolers and distributed hundreds of coolant vests and refrigerant packs that workers wear around their necks to prevent heatstroke.

Moves to improve conditions came after Takeshi Tanigawa, a medical advisor to Tepco, had warned that nuclear power plant workers faced an increased risk of accidents due to sleep deprivation and fatigue.

"Their level of stress is unimaginable," Tanigawa, a professor of public health at Ehime University, said, adding that, without counseling, some were at risk of developing post-traumatic stress syndrome.

Goshi Hosono, an adviser to the prime minister, acknowledged that the government had failed to adequately protect the workers. "In our early response, we did not have a system in place to manage radiation risks," he said.

Last week, Tepco acknowledged that the risk of radiation exposure and concern among relatives had added created "multiple" sources of stress for workers.

The firm said it had introduced regular health checks and that medical staff would be sent to the site to offer counselling for between two and four days a month.

The Fukushima operation's dependence on hundreds of contract workers was highlighted in May when Japanese media reported that two men who replied to a wanted add for truck drivers in northern Japan - with a daily wage of 12,000 yen - were sent to work at the nuclear plant at short notice.

The notice had been posted in Kamagasaki, a slum area of Osaka where day labourers and the unemployed have been hired out to small construction firms, often with the help of yakuza go-betweens, since the 1960s.

Japanese media reported that the daily rate for labourers is three times higher than usual for people willing to work inside the plant's grounds, and 1.5 times higher for jobs in the wider nuclear exclusion zone.

But, according to one report, some ads for plant workers offer 10,000 yen a day for truck drivers, with jobs as security guards paying as little as half that.