Critics slammed the move as a bad omen for plans to get the new reactors producing much-needed electricity by July 1, 2018.
But Energy Minister George Smitherman said it was necessary because bids on a nuclear plant are complicated and the bidders need more time to consult with the government on their efforts.
Bids had been due Oct. 1 under guidelines set out this spring.
The new deadline is Dec. 31, by which time the province had promised it would make a decision on whether the contract would go to Ontario-based AECL, French nuclear giant Areva NP, or Westinghouse.
Smitherman, who took over the energy portfolio in a cabinet shuffle last month, is now promising a decision by the end of March 2009.
Opposition critics warned this could be the first of many delays as the province looks to boost nuclear generating capacity after coal-fired power plants are closed in 2014.
"It's an ongoing habit of this government to continually put things off," said Progressive Conservative MPP and energy critic John Yakabuski (Renfrew-Nipissing-Pembroke), noting the Liberals have already broken promises to close the coal plants in 2007 and 2009.
"This delay is just symptomatic of what one often sees with nuclear power," added New Democrat MPP and energy critic Peter Tabuns (Toronto-Danforth), citing numerous delays in building the original Darlington plant and refurbishments of other reactors in Ontario.
The delay should not affect plans to have construction on the new reactors at Darlington begin in 2012 and be ready for operation six years later, said Diane Flanagan, of Infrastructure Ontario, the provincial agency in charge of the nuclear procurement effort.
But anti-nuclear activist Shawn-Patrick Stensil of Greenpeace questioned whether the delay would slow other parts of the process, such as the winning bidder getting a construction licence from the Canadian Nuclear Safety Commission in 2009.