Former Ontario premier Dalton McGuinty has testified under oath that it was his decision to cancel two contentious gas plants, a move that cost taxpayers hundreds of millions of dollars.

Appearing before a legislative committee on Monday morning, McGuinty said when he made the decision to cancel the projects in Oakville and Mississauga, he did not know what it would cost.

"I knew that going into this, that when I said we're going to relocate the gas plants, that I did not have at my hand the costs associated with that," he said.

The most recent figure attached to relocating the two plants to Napanee and Lambton has soared to an estimated minimum of $585 million, far above the $230 million McGuinty and the Liberals had been claiming.

An auditor general’s report pegged the cost of cancelling the Mississauga gas plant during the 2011 provincial election campaign at $275 million, $85 million more than McGuinty admitted when he was premier.

In April, the Ontario Power Authority said the cost to cancel the Oakville plant was up to $310 million, more than seven times the $40 million-figure the Liberals had been using.

“I know there’s been a tremendous amount of confusion around the numbers,” McGuinty said.

The former Ontario premier said he regrets that it took his government so long cancel the projects, however, he pointed out that all three provincial parties agreed with the decision to relocate the plants.

“All three parties recognized the people of Mississauga were right and my government was wrong,” he said, adding that the site of the Mississauga plant was located in a residential area, close to a hospital and condo towers.

Asked if he had a “top end” dollar figure that he was not willing to surpass when making the decision to cancel the plants, McGuinty said: “Parents attach a pretty high price to the health and well-being of their children.

“We offset those (costs) against the rights of families in those two communities.”

Throughout his testimony, McGuinty maintained that the decision to cancel the plants was the right thing to do.

“The cost to relocate these plants is higher than anyone would have wanted. Our ability to get the numbers out in a timely way has been less than stellar, and we have struggle to produce documents in a timely way, all of this is true,” he said. “But I strongly believe locating gas plants in those two communities was wrong, and relocating them was right.”

The Liberals came under fire for cancelling the Mississauga plant two weeks before the 2011 provincial election, which opposition parties argue was done to save five Liberal seats.

Asked about the timing of the decision, McGuinty said his government believed, for some time, that the plant would not go forward.

“The procurement had been made in 2004. There was some challenges with finance. People in the community thought it wasn’t going to go ahead,” McGuinty told reporters during a press conference on Monday.

When Ontario Liberal Premier Kathleen Wynne appeared before the committee last week, she was criticized for being evasive in some of her testimony.

For her part, Wynne said it was her promise to help the opposition parties get all the answers on the cancelled gas plants that cleared the way for McGuinty's testimony.

"I wanted us to open up the process and make sure the committee could ask all the questions they wanted, and that included having the people that they wanted to hear from come to the committee," she said.