The opposition hammered the Conservatives Wednesday over an imminent investment treaty with China, with the NDP and Liberal leaders both demanding a chance to examine the bill in committee before it is approved.

The Foreign Investment Promotion and Protection Agreement is likely to become law soon after Thursday, when a 21-day period in which the agreement had to be before Parliament ends and the deal can be signed off by cabinet. Once both countries ratify the agreement, it becomes law.

Critics say the deal should have gone before a parliamentary committee to be examined by MPs and experts for both its advantages and weaknesses.

The Conservatives, meanwhile, say the agreement gives Canadian investors equal protections in China to those afforded to Chinese investors in Canada. And they say the agreement has received "100 per cent" support from Canada's business community.

New Democrat Leader Thomas Mulcair asked Wednesday why the Conservatives had refused to allow such an important bill to be studied in committee, and vowed an NDP government would overturn the 30-year agreement.

"Let me be very clear, the Conservatives won't tie the hands of the NDP. We will revoke this agreement if it is not in the best interest of Canadians. Why won't the prime minister allow the study of this bill, what does he have to hide?" Mulcair demanded during question period in the House of Commons.

Harper didn't address Mulcair's question about studying the agreement in committee, but instead took aim at the NDP's economic policy.

"The leader of the NDP says he would revoke the hard-earned right of Canadian investors to be protected in a marketplace like China," Harper said.

"That is precisely why, of course, Canadian investors and the business community and the public at large does not trust the NDP with economic policy. We know China is an important marker, we know that Canadians need protection there, this agreement does that."

Liberal interim leader Bob Rae also joined in the fray. He asked Harper to explain why the trade agreement has a 15-year notification period for cancellation, pointing out that most similar deals have a six-month or one-year notification period.

"I wonder if the prime minister can explain that and would he not agree that in order to clear up the issues that have been raised...would he not agree that a committee hearing would be a very good way to let expert witnesses appear to answer some of these questions," Rae said.

Harper once again declined to answer the direct question, instead discussing the value of the agreement and suggesting the government was committed to pushing it through.

"We are achieving for the first time long-term protection for Canadian investors in China and that's an important thing for this country -- Chinese investors have long been able to expect that there will be lawful protections here in this country, and it's important we get the same," Harper said.

Canada and China signed the agreement at the beginning of September, and the government tabled it in the House of Commons a few weeks later. Under new rules set by the Conservatives, the agreement must be before Parliament for 21 sitting days before it can be ratified.

That period runs out on Thursday, at which time it appears all but certain to be approved.

Critics say the deal should have gone before parliamentary committees to be examined by experts for its implications as well as for flaws and weaknesses.

"We have a government that is refusing normal, democratic process," said NDP industry critic Peter Julian.

Green Party Leader Elizabeth May argued that the deal would give Chinese corporations, and by extension Beijing, new powers to influence Canadian policy, in areas ranging from investment and industrial development to the environment and health.

And since Canada's trade in China is about half that which Chinese investors bring to Canada, Canadian governments at all levels will have little choice but to cater to the whims of a China desperate for natural resources, she said.

"We become the resource colony in that context," May told The Canadian Press.