(Reuters) - The number of workers at Japan's Fukushima nuclear plant on false contracts has increased in the last year, the station operator said, highlighting murky labor conditions at the site despite a pledge to improve the work environment.

The survey results released by Tokyo Electric Power Co (9501.T) (Tepco) late on Thursday showed that around 30 percent of plant workers polled said that they were paid by a different company from the contractor that normally directs them at the worksite, which is illegal under Japan's labor laws.

A Reuters report in October found widespread confusion among plant workers at the Fukushima facility over their employment contracts and their promised hazard pay increase.

Many workers asked Tepco in the survey forms whether they were supposed to receive an equivalent of about $180 a day in hazard pay, the company said, adding that it did not mean each worker would necessarily see a pay increase of that amount.

Tepco said last November it would double the allocation for hazard pay to workers at Fukushima.

The company's latest survey showed that more than 70 percent of workers did receive some explanation about a pay rise in the past year, while some 1,400 workers out of 2,400 that responded to the question said they did receive an increase in pay. Workers were not asked to detail their pay rise or when they received it.

The Japanese government and Tepco vowed last year to improve working conditions at the plant, where sub-contractors supply the bulk of workers conducting a cleanup after the worst nuclear disaster since Chernobyl in 1986.

Tokyo Electric, known as Tepco, said a questionnaire sent to thousands of workers at the plant indicated 30 percent of them were on false contracts, an increase of 10 percentage points since it carried out a similar survey in 2013.

The utility survey covered 6,567 contract workers at the station north of Tokyo and about 70 percent of them responded.

It did not question its own employees, who form a small part of the huge workforce on the clean up that is expected to take decades and cost tens of billions of dollars.

(Reporting by Aaron Sheldrick; Editing by Jeremy Laurence)