TORONTO - Premier Dalton McGuinty is "selling out'' Ontario by refusing to ensure that most of the nearly $1.5 billion promised for infrastructure will be spent in the province, despite a last-minute pledge to buy "Canadian content'' in public transit, opposition parties charged Thursday.

All transit vehicles bought with provincial funds must have "at least 25 per cent Canadian content,'' Transportation Minister Jim Bradley said Thursday after McGuinty announced an extra $1 billion for municipal infrastructure.

Bradley wouldn't divulge the details of what constituted "Canadian content,'' but insisted that the plan would help boost the province's economy and appease any trade partners who might retaliate by cutting down on Ontario exports.

And most taxpayer dollars allocated for public transit are already being spent in the province, he said.

But NDP Leader Howard Hampton denounced the Liberal plan as doing little for the province's hard-hit manufacturers.

"Twenty-five per cent content is not going to do the job,'' said Hampton, whose party has been pushing for the Liberals to adopt a more rigorous "buy Ontario'' plan.

"We could easily right now attain 50 per domestic content, which would not only sustain but create other good manufacturing jobs in the province. The McGuinty Liberals, as usual, are taking the lowest common denominator.''

The plan came on the heels of McGuinty's announcement that another $1 billion would be spent this year to help repair and build roads and bridges as well as improve public transit and affordable housing.

The money adds to the $300 million that was earmarked for municipal infrastructure in the government's fall economic statement and another $150 million announced last month by McGuinty.

"You just can't grow this economy unless you make substantive investments in infrastructure,'' McGuinty said after unveiling 22 new double-decker buses for GO Transit in north Toronto.

McGuinty, who's come under fire for massive job losses in the province's hard-hit manufacturing sector, defended his government's decision to invest in the double-deckers, which were built in Scotland.

"I think Scotland's the only place that makes these buses,'' McGuinty said after touring one of the buses.

"If we start saying that the only place that we're going to buy from - exclusively - is within Canada or within Ontario, then what if the States and other parts of the world were to match that? Then they'd stop buying our bi-level rail cars from Thunder Bay and those people would be out of work.''

While McGuinty was climbing aboard Scottish-made buses, the Liberals were voting down the NDP's "buy Ontario'' bill in the legislature, delivering a "double slap in the face'' to laid-off workers, Hampton said.

The bill would require municipalities and transit authorities to give preference to mass transit vehicles whose final assembly is done in Ontario, and where at least half of the total dollar value of the purchase is attributable to Canadian-made parts and labour.

The NDP argued it was high time to revive such programs given the province's troubled economy, but the proposal was dismissed by McGuinty, who had hinted the Liberals had something similar in the works.

While provincial revenues will be coming in "at a slower pace,'' the billion-dollar boost for infrastructure will enhance productivity and create an estimated 10,000 new construction jobs, McGuinty said.

And despite the flurry of pre-budget spending announcements in recent weeks, McGuinty maintained his government will continue to deliver balanced budgets even though the province is grappling with a slowing economy.

"What we want to do is be prudent and be responsible in terms of how we invest limited public dollars, and you've got to invest them in a way that makes a difference,'' he said.

But Opposition Leader Bob Runciman dismissed the additional money as a "billion-dollar Band-Aid.''

McGuinty wouldn't say how the province will disburse the extra funds for infrastructure, deferring the details for the province's March 25 budget.

But not all of the money is new. It includes $100 million for repairs on 4,000 affordable housing units that McGuinty announced Monday.

The new funds will also add to any money that may flow from Finance Minister Dwight Duncan's proposal to give a portion of the province's future surpluses over $800 million to municipalities.