The Canadian Union of Public Employees is suing the Ontario government over the sale of Hydro One, the province's electricity transmission monopoly.

CUPE president Fred Hahn says the union's lawyers served the Ministry of the Attorney General on Tuesday with notice of intent to sue, but the government said today it still was "not aware of any action from CUPE."

CUPE claims the Liberals inappropriately mixed government and party business by holding fundraisers with cabinet ministers for up $10,000 a plate, including one attended by bankers who allegedly profited from the sale of Hydro One shares.

The union says it's using the lawsuit to argue that shares of the electrical transmission utility should remain in public hands and that the sell-off, which began last year, is opposed by most Ontarians.

"It seems remarkable to me that a provincial government would act in a way that is so broadly unpopular," said CUPE Ontario president Fred Hahn at a morning news conference at Queen's Park.

The Liberals plan to sell up to 60 per cent of the huge utility, which also serves as a local electricity distribution company for 1.3 million homes, mainly in remote and rural parts of Ontario.

With today's announcement, CUPE is serving the province with 60 days notice of its intention to file the lawsuit. The union says it can't get into specifics about the lawsuit until it's officially filed in November.

The province raised $3.2 billion from selling 30 per cent of Hydro One so far.

The Liberals say the money raised from the privatization of Hydro One is needed to fund public transit and infrastructure projects and to pay down debt.

However, Hahn said, the move is not in the best interests of the province.

"This is one piece of a much larger chorus of opposition that this government needs to pay attention to," said Hahn. "We have firm belief that we can stop them."