MISSISSAUGA, Ont. - Criticism that Ottawa's economic-stimulus plan has failed to deliver jobs is wrong, Prime Minister Stephen Harper said Thursday as he ended months of indecision by announcing a seven-month extension to the multibillion-dollar program.

Harper called the job-creation results of the program "extremely clear" and said a critical report this week by Kevin Page, parliamentary budget officer, "lacks credibility."

"This has been -- everybody knows -- an extremely successful program," Harper said. "There's been strong job creation."

Harper's remarks came after he announced the program's deadline had been pushed back from its original date of March 31 by a full construction season to Oct. 31, 2011.

The prime minister insisted more than 90 per cent of projects would be completed by the end of next March but the extension was warranted for the "small" number of projects hit by bad weather or "Murphy's Law."

The federal government has been obfuscating for months on the deadline.

After insisting until just a few months ago there would be no extension, officials began quietly letting it be known they were looking for a way to ease up without penalizing municipalities that had pulled out all the stops to finish their projects on time.

Earlier Thursday, Page said more than $300 million might not be spent on infrastructure stimulus as planned unless the government quickly clarified its spending rules.

And a survey commissioned by Page this summer suggested that of the municipalities that received Infrastructure Stimulus Fund money, 33 per cent said they saw an increase in employment as a result of the funding.

Twenty-one per cent said there were fewer jobs, and 43 per cent said they saw no net impact whatsoever.

The federal government has committed $16 billion toward 23,000 roads, bridges and other infrastructure projects as part of a stimulus plan first announced in February 2009 in the depths of the economic slump.

Pushing back the deadline won't cost federal taxpayers any extra money, Harper said.

Only those projects already underway and which incur eligible costs before March 31, 2011 will be granted the extension.

Responding to Page's view that the program has proven extremely costly in terms of the employment it has created, Harper bragged the stimulus plan has created 420,000 net new jobs.

"Canada's economic action plan is working," he said after touring an unfinished college expansion.

The Federation of Canadian Municipalities welcomed the extension to the March deadline.

"Municipalities have worked flat out to keep 90 per cent of projects on schedule," said federation president Hans Cunningham.

"Now they'll have one more construction season to turn every last stimulus dollar into a job."

Ontario Infrastructure Minister Bob Chiarelli said one more construction season made sense.

"We've said for some time, it would not make sense to build four walls and not the roof," Chiarelli said.

Both the federal Liberals and New Democrats took credit for the Conservative government's change of heart, saying they had been calling for the extension for months.

The Liberals said the delay cost municipalities money in overtime and higher prices for construction materials.

The extension was also praised by the group speaking for Canada's largest colleges and institutes.

"The announcement . . . enables our members to complete the expansion and modernization of their infrastructure in a reasonable and fair manner," said John Davies, chairman of Polytechnics Canada.