The ruling and opposition camps grappled on Monday over a proposal to expand an overtime work system, with the opposition suggesting the government deliberately manipulated a survey to make it look like the system could improve the nation’s chronic overwork problem.

Monday’s session of the House of Representatives Budget Committee was briefly halted amid heated debate after labor minister Katsunobu Kato apologized for the “inappropriate” survey in 2013, which he said compared data measured using two different methods.

Prime Minister Shinzo Abe had used the data in the same committee late last month in support of expanding the discretionary labor system. After the reliability of the data came into doubt last week, he retracted his remark and apologized.

Under the system, employees are given a fixed number of overtime hours and are paid on the assumption that they worked those hours, meaning any further overtime is unpaid.

The system is only applicable to certain fields, but the proposed working practices reform bill would expand its scope.

The government says that this would lead to a more flexible working environment and that business lobbies support the move. But labor unions say that it could worsen the overwork problem, which has long produced cases of karoshi (death by overwork).

The government had planned to submit the bill to the Diet later this month, but it will now likely be delayed until March. On Monday, however, six opposition parties agreed to stand against the bill’s submission.

The problematic 2013 survey on working hours by the Health, Labor and Welfare Ministry concluded that the average worker on a discretionary labor contract generally works shorter hours than one on a conventional contract.

But the survey asked workers under the discretionary system how many hours they worked per day, while the conventionally employed workers were asked to recall the most hours they had worked in a single day in the past month. The number of overtime hours worked on that day was added to the legal working day of eight hours to come up with the figure.

Chief Cabinet Secretary Yoshihide Suga, the government’s top spokesman, admitted at a news conference that the use of different survey methods was “highly inappropriate.”

Akira Nagatsuma, acting leader of the Constitutional Democratic Party of Japan, the largest opposition force in the Lower House, said at a meeting that he suspects the ministry intended to make working hours seem shorter among the discretionary workers in anticipation of the opposition’s assertions to the contrary.

Yuichiro Tamaki, leader of the opposition party Kibo no To (Party of Hope), said during a visit to Fukushima Prefecture that the changes proposed to the discretionary system should be removed from the bill altogether.

A senior ministry official told reporters that the ministry’s analysts “were not aware they were comparing different (data)” when compiling the results.

“There is no way they intentionally made up the figures,” the official said.

But the official said the report was generally sloppy, with at least three instances of simple errors, including 45 overtime hours recorded in a single day.

Kato said he was aware of the flaws on Feb. 7 and vowed to look into the details of why the comparison was made with data measured using two different methods.