OTTAWA - The NDP is calling for an emergency debate on the Harper government's controversial census changes.

Party leader Jack Layton sent a letter to the Speaker of the Commons today asking for a debate on the "integrity of the census" when the House resumes next month.

He made the call as the Commons industry committee met to discuss the census issue.

The Conservatives have been facing heavy criticism since they announced in July that they are making the long-form census voluntary.

They argue that the survey, which has been mandatory, is too intrusive.

But experts say a narrower cross-section of society will participate in any voluntary survey, making the data less representative and incomparable to previous census surveys.

"The ability to change the long form diminishes with each passing day, as printing deadlines loom and preparations by Statistics Canada proceed," Layton wrote in his letter.

"At stake, of course, is the continuity and reliability of the country's historic data on which so many Canadians and their institutions depend."

Last week, the government blinked in the face of a francophone group's court challenge, but it's still refusing to bend on its overall plan to scrap the mandatory long-form census.

Industry Minister Tony Clement announced last Wednesday that the government will add three questions on language skills to the obligatory short-form 2011 census. In the past, the questions have been part of the long-form census.

Clement's move was aimed at short-circuiting a court challenge by Federation des communautes francophones et acadiennes du Canada. The group maintains that scrapping the mandatory nature of the long-form census would violate the Charter of Rights and the Official Languages Act.

Earlier Wednesday, the Federal Court agreed that the situation is urgent and set a Sept. 27-28 hearing date for the federation's case.

Clement also announced that the government will introduce legislation this fall to remove the threat of jail time for people who refuse to fill out any mandatory federal survey.

Clement said the changes are as far as he's prepared to go to mollify critics of the census changes.

All three opposition parties have been calling on the government to eliminate the threat of jail time and reinstate the mandatory nature of the long-form census.