TORONTO - Gas prices in Ontario would be regulated under an NDP government to stop gouging at the pumps, Leader Andrea Horwath said Thursday.

"Filling your tank should not be a game of chance," said Horwath.

"We can't fix everything, but we can take steps to protect households from gas price gouging, and I think it's time that we do that."

If it wins the provincial election this fall, the NDP promises to set a weekly price cap on gasoline to give consumers some predictability.

Over the last year, gas prices in Ontario have climbed about 30 cents per litre, and are spiking overnight by as much as 10 cents a litre, said Horwath.

Her plan, she added, doesn't mean consumers will see a difference on a daily basis, but it will remove volatility and should make gas cheaper on an annualized basis.

"Independent experts estimate that gas gouging accounts for a big chunk of the price that people pay at the pump; as much as 25 cents a litre in one week this past May," Horwath told reporters outside Ontario's legislature.

"Other provinces have similar plans in place and the result is a dependable, reliable price."

The NDP plan would give the Ontario Energy Board, which already regulates electricity and natural gas, the power to set and monitor caps of gasoline.

No new funding would be required for that agency, however, because the watchdog already has the budget and staff needed to do the job, Horwath said.

Liberal campaign co-chair and former finance minister Greg Sorbara said there was no clear evidence that a system of regulation would lower prices, adding there is little a provincial government can do to regulate prices that are set internationally.

"On the surface it sounds interesting, but the fact is that where there is regulation of the price, gasoline costs more," Sorbara said.

"What's of interest to the people of Ontario is not that it will be regulated, but that the price come down."

The Liberals don't have any plan to regulate gas prices themselves, Sorbara added.

"If there were evidence that a system of regulation could actually get the price down, it's something that we would have to look at, but right now there's no evidence to that effect," he said.

There is some form of price capping or regulation in 24 U.S. states, as well as in Quebec and all the Atlantic provinces.

According to the gas price comparison website GasBuddy.com, prices in Ontario averaged 127.7 cents per litre on Thursday, New Brunswick averaged 124.2 cents per litre and Quebec was at 133.5.

The NDP has already promised to restore a 14 per cent corporate tax rate, reversing a plan by the governing Liberals to slash those rates to 10 per cent.

The party has also promised to scrap the HST from hydro and home heating bills, but Horwath wouldn't say Thursday if she would take the tax off gasoline.

More details, she added, will come when her party unveils its platform at the end of the month.