Norfolk County Council has signed a contract for a £500m waste incinerator in King's Lynn.
The deal, which took a year to finalise, commits the council to work with developer Cory Wheelabrator at the Willows Power and Recycling plant.
A consultation into plans for the scheme is under way after concerns were raised over its impact on health and the environment.
The Conservative-controlled council had agreed the contract in March 2011.
The deal signed on Tuesday sets out how much waste the council will provide the plant at Saddlebow, as well as how much it will pay for it to be processed.
The Borough Council of King's Lynn and West Norfolk, Norwich City Council, some local MPs and campaign groups have condemned the project.
Residents have voiced concerns over noise, dust, smell, air pollution and health issues in relation to the site at Willows Business Park.
Nick Daubney, leader of the Borough Council of King's Lynn and West Norfolk, said he was surprised the deal had been signed already.
"I was deeply disappointed and somewhat surprised it was signed behind closed doors."
Planning permission has yet to be granted for the project, which would process about 260,000 tonnes of rubbish every year.
The council said the plant would save £8m a year that would otherwise be spent sending rubbish to landfill sites.
Having finalised the deal, the county council could have to pay Cory Wheelabrator up to £20m if it does not get planning permission for the incinerator.
Mike Knights, vice chairman of the campaign group King's Lynn Without Incinerator, is calling for the planning decision to the secretary of state for communities and local government.
"The county council have made their intention clear from the outset that they intended to sign up for this incinerator.
"Now they have formalised their position and have demonstrated they can't make a fair, impartial planning decision."
Bill Borrett, the county council's cabinet member for environment and waste, said the financial agreement was separate from the planning process.
"The planning process is a national process and, as such, is separate to the wants and needs of the county council."