A multimillion-dollar, two-year facelift will modernize Quebec's Honoré Mercier Bridge without completely disrupting commuter traffic, transport authorities promised Monday.

About 28 million vehicles per year use the bridge, which connects the Montreal borough of Lasalle to the Mohawk reserve at Kahnawake. Part of the bridge spans Mohawk territory.

It's estimated the overall renovation will cost $100 million, which would make it the most expensive bridge repair work in Canadian history.

The renovation will replace the bridge's main apron, also known as the deck, which will extend the structure's lifespan by at least 75 years, federal and provincial transport authorities said at a news conference in Chateauguay, Que., on Monday.

The first $66-million renovation phase will involve replacing the 1.3-kilometre apron one section at a time, with prefabricated concrete deck panels.

Federal and provincial transport authorities say the renovations are the result of a "historical" agreement with the Mohawk Council of Kahnawake, whose land is bridged by the structure.

As a result, the Mohawk Bridge Consortium — which includes five Mohawk companies — will handle the first phase of work in what can be considered a "fair deal," said Glen Carlin, director of the Federal Bridge Corp. Ltd.

"We can affirm today that the price we've received is a fair and reasonable price," he said at the news conference.

The second phase, which will involve redoing the deck's surface, will undergo a public tender process in the fall.

Some of the construction work will start this summer but the bulk of planned renovations won't start until spring 2009, which is when authorities say motorists will most feel the effects.

Two lanes in each direction will remain open during rush hour, with one lane open each way during slower periods, to allow some traffic through, the Quebec Transport Ministry said.

It's impossible to completely close down such an important bridge, it said.

Pinching off several lanes will have a huge effect on South Shore traffic, said Yves Desautels, the Montreal traffic reporter for the CBC's French-language service, Radio-Canada.

"When you leave just one lane open in each direction, like it happens sometimes during the weekend, it causes monster congestion," he said on Monday. "So the impact will be significant next year."

The federal and provincial governments are sharing the renovation costs.

The work is expected to create 1,000 direct and indirect jobs. The work should be completed by 2011.