Imperial Oil says it's too early to tell if supply will be affected after a power outage triggered a shutdown at its Nanticoke refinery on Friday.

Nanticoke accounts for about 25 per cent of the company's total refining capacity. The cause of the outage is being investigated.

The shutdown was site-wide, and the refinery has since been restarted, which can take between three-to-five days, said company spokesman Pius Rolheiser.

He said none of the refinery's units was damaged from the outage and no employees were injured in the incident.

"It's too early to know whether there will be any impact on supply," Rolheiser said.

He added Imperial Oil also has the option to draw from its Sarnia refinery, which is a similar operation to Nanticoke.

The Nanticoke refinery can process 118,000 barrels of oil a day, while Sarnia can process 121,000 barrels.

A fire at the Nanticoke refinery in mid-February crippled production in Ontario and drove up gas prices.

Many Esso stations ran out of fuel entirely, which led to shortages at competitors Shell and Petro-Canada.

Drivers saw gas prices soar from the mid-80-cent range.

Any shortage resulting from Friday's shutdown could drive up gas prices, which is already well above the $1-a-litre mark.

"It's certainly possible," said Liberal MP Dan McTeague, the consumer affairs critic and longtime critic of the oil industry.

McTeague said the refinery restart must be done in a hurry to avoid a three-to-four cent increase.

"We saw prices move from 85 to a dollar when Nanticoke was out -- it was supposed to be only for a few days, it turned out to be six weeks, if you could find product -- so this is a very serious situation," he said.

Industry analysts expect gas prices to increase anyway because the busy summer driving season is approaching.

A report by the Canadian Centre for Policy Alternatives last week said Canadians are being gouged at the pumps across the country by as much as 27 cents a litre.

The study found an unjustified hike in prices after Hurricane Katrina when taking into account the cost of crude oil, production and profit margins before the devastating hurricane.

Opposition parties demanded the federal government take action following the report.

The Competition Bureau, meanwhile, says there is no proof of collusion in Canada's oil industry.

With a report from CTV's Roger Petersen and files from The Canadian Press