Gasoline retailers in four Quebec regions have been accused of price fixing, the federal Competition Bureau announced Thursday.

The alleged scheme involved 13 people and 11 companies in the Quebec regions of Sherbrooke, Magog, Victoriaville and Thetford Mines, Commissioner of Competition Sheridan Scott told reporters at a Montreal news conference.

Three companies -- Les Pétroles Therrien (operating under the name Petro-T), Distributions Pétrolières Therrien and Ultramar -- pleaded guilty Thursday in Quebec Superior Court in Victoriaville. The court handed out fines totalling more than $2 million to the three companies.

Former Ultramar employee Jacques Ouellet also pleaded guilty and was fined $50,000.

The investigation began two years ago.

"Gas retailers phoned each other to agree on the price they would charge customers for gasoline," Scott said. "An overwhelming majority of retailers (in those regions) participated."

Ultramar is the only major oil company implicated in the investigation. Many of the others were local franchisees operating under the Esso and Shell brands.

Scott said there is no evidence Ultramar's head office was directly involved in the scheme, which cost the company a $1.85-million fine.

"This is obviously a regrettable situation that we deplore,'' Christian Houle, an Ultramar vice-president, said in a statement posted on the company's website. "We believe these employees did not mean to cause harm but acted out of carelessness without taking into consideration established rules and consequences of their actions."

Houle said Ultramar chose to plead guilty to avoid a long and costly trial.

The company's fine was one of the largest the Competition Bureau has ever handed out for price fixing, Scott said.

Scott said price fixing remains illegal and that her organization plans to take action whenever possible to expose the practice.

"Price fixing is a fraud against consumers," she said. "Price fixing deprives Canadians of the benefit of competition."

In some regions of Quebec, gas is selling for as much as $1.51 per litre.

Wiretaps, searches and testimony were used as evidence in the case.

An investigation into potential price-fixing in the retail gasoline market continues in other markets in Canada. According to Liberal MP Dan McTeague, a vigilant observer of gas prices, smaller cities are more likely to see this sort of conspiracy because it's easier to coordinate a small group of retailers.

"In smaller markets, you can imagine 15 or 16 stations might have 60 per cent of the market," he told CTV Newsnet on Thursday.

With files from The Canadian Press