Liberal MPs are demanding the federal government immediately begin the process needed to replace the degrading Champlain Bridge near Montreal.

The party's interim leader Bob Rae says the Harper government has spent five years dithering instead of getting the aging bridge on a fast-track to being replaced.

The bridge, which crosses the Saint Lawrence River and the Saint Lawrence Seaway canal system, has been a source of public concern since the release this spring of a report that warned it's at risk of collapse.

This week, federal Transport Minister Denis Lebel refused to release another report analyzing the safety of the Bridge, saying he didn't want the public to worry unnecessarily. But his comments had the opposite effect: His remarks further fed local concern.

So on Wednesday, in a surprise about-face, the government decided to release the Pre-feasibility Study Concerning the Replacement of the Existing Champlain Bridge, written in February 2011.

The report says it would cost as much as $25 million per year over the next decade simply to prolong the life of the bridge. It said even with that amount of annual maintenance, it wouldn't be enough to improve "the level of seismic performance" or rehabilitate the bridge deck.

Rae and Quebec MP Denis Coderre, who is the Liberal party's Transport, Infrastructure and Communities critic say the time to get started on a process to replace the 49-year-old bridge is now.

"The question is not whether or not a new bridge is needed, but rather when the working group's proposals will be submitted," Rae said in a statement Thursday.

Later, he told reporters that the government has other reports that have not been made public.

"Even now we do not have all the information which is at the base of these reports," said Rae. "...The fact remains that we have wasted five years -- five years in which work has not been done."

Coderre said even if the government were to decide today to replace the span, it would take years to complete mandatory environmental impact studies before construction actually began.

With about 160,000 crossings a day, the Champlain Bridge is the busiest bridge in Canada. Rae says those who use the bridge need to be assured it's safe.

"Champlain Bridge users have had enough," concluded Rae. "They have the right to know the condition of the infrastructure they use every day, if the road they travel every morning and evening is safe and, above all, they have the right to know the government's time frame for finally building a new Champlain Bridge."

A new bridge is expected cost about $1.3 billion. An alternate plan, to build a tunnel under the rivers and canal would cost $1.9 billion. Both estimates include the $155-million cost of demolishing the existing span.