MONTREAL - There is now unprecedented political pressure on the Quebec government to call a public inquiry into allegations of corruption in the construction industry, with Premier Jean Charest's Liberals finding themselves increasingly isolated.

Charest's last major remaining ally -- the Quebec Federation of Labour -- has bailed on him.

The province's largest labour group, previously opposed to the idea of an inquiry, has done an about-face and is now publicly demanding one, while also issuing a highly personal plea for Charest to change his tune.

"After the events of the last two years, the discussions we've had among ourselves, the reports we've seen on TV and radio, we've decided together to demand, from the government of Premier Jean Charest, a commission of inquiry," union president Michel Arsenault told a news conference Wednesday, surrounded by colleagues.

"We believe the time has come for one."

The premier has refused to call a probe into corruption allegations involving the construction industry, politically well-connected businessmen, and crime groups like the Mafia.

Charest says police investigators should be allowed to do their work without the spectacle of a costly public probe. The police's Operation Hammer, ordered by the Charest government, has made a handful of arrests.

But the premier's refusal to call an inquiry has cost him almost all of his political capital, with his once-popular Liberals seeing their public support crater.

The only major voices supporting Charest were the province's municipalities and the labour federation, which represents construction workers. They have both abruptly changed their tune in recent days.

"Our members, and the general public, are unsatisfied by the slowness of the police investigations," Arsenault said.

"Every day, there's something new. It's becoming almost a reality show -- this whole thing."

Charest's public security minister, Robert Dutil, made a plea Wednesday for Quebecers to be patient.

"It's better to have an investigation that's properly done and properly put together to be sure that we have accusations and results, such as criminal convictions," said Dutil, chosen by Charest's Liberals to speak Wednesday on behalf of the government.

Parti Quebecois Leader Pauline Marois said Charest's unwavering refusal hold an inquiry raises questions.

"I'll repeat the question: What is Mr. Charest afraid of to not accept this request (to hold an inquiry), which is becoming unanimous?" Marois asked.

Public calls for an inquiry into political corruption are getting louder and even more unwieldy, as allegations surfaced this week against Laval Mayor Gilles Vaillancourt.

Vaillancourt allegedly offered envelopes packed with cash to PQ candidate Serge Menard in 1993 and provincial Liberal hopeful Vincent Auclair in 2002.

The PQ wants Charest to remove Vaillancourt from the mayor's office and the federal Conservatives have called for Menard, now a Bloc Quebecois MP, to step down.

Charest refused Wednesday to step in and take mayoral duties away from Vaillancourt, who vehemently denies the accusations.

In recent days, investigative reports have revealed close ties between construction-industry executives, union bosses, and people closely linked to the Rizzuto crime family.

It was similar reporting last year, especially by Radio-Canada, that caused a municipal contracting controversy in Montreal to erupt into a full-blown political scandal that rocked Quebec.

The French-language CBC reported that Mob-linked companies colluded under a system where competitors would face threats if they bid without permission on public contracts. The system was allegedly designed to keep the price tag on projects artificially high.

By setting up what essentially operated as a cartel, a group of companies called the "Fabulous 14" is accused of driving up the price of public contracts in Quebec by as much as 35 per cent.

"FTQ (QFL) members are Quebec citizens," Arsenault said. "They are taxpayers.

"If it's true that we're paying too much for our infrastructure, that is fundamentally unfair to the public -- to our members."