The row over controversial plans to build an incinerator in West Norfolk has taken a fresh twist after leaked internal Tory documents appeared to show that the ruling Conservative group was made privy to officer advice which was not shared with other councillors.

The county council's ruling cabinet agreed in March to press ahead with a deal to award the contract for the �600m energy for waste facility to Anglo-US firm Cory Wheelabrator.

But the issue has sparked massive local opposition which saw environment secretary Caroline Spelman recently raise concerns if the project could demonstrate the broad consensus of support needed to secure �169m of PFI credits from the government.

Now opposition leaders have accused the Conservatives at County Hall of agreeing in secret to press ahead with the incinerator plans, three days before the cabinet decision.

They point to internal minutes of the meeting which state that following a lengthy debate about the waste PFI contract a "show of hands clearly indicated that the group was, in principle, in favour of energy to waste incineration".

Critics believe that this proves the cabinet decision was essentially a rubber stamping exercise and the real decision had been made by the group.

But council leader Derrick Murphy said the accusation was nonsense and accused anti-incinerator campaigners of getting desperate.

Anger has also been stirred by a deal to let members of the cabinet from West Norfolk divisions abstain from the decision which, the minutes said, was due to a "conflict of interest (ie the best interests of their division versus the best interests of Norfolk as a whole)".

While the minutes also show how two West Norfolk Tory rebels John Dobson and Brian Long opposed to the incinerator plans, tried unsuccessfully to get a decision postponed until after the local elections on May 5.

But there are questions over whether the most damaging revelation was that deputy leader Ian Mackie then gave the group a detailed briefing on the potential implications and commercial risks of three outcomes; proceeding with the cabinet vote, deferring any decision until after the elections, and abandoning the scheme entirely, produced in conjunction with the council's heads of law, finance, and procurement, as well as solicitors Sharpe Pritchard.

Green councillor Andrew Boswell said he was concerned that members of the ruling group had been made privy to advice from officers which had not been made available to other councillors, including members of the scrutiny committee.

"This takes the decline of openness and transparency at Norfolk County Council to a new low," Dr Boswell said. 'Once again, a decision has been made by a small clique of councillors, who have withheld key information. Scrutiny councillors had no chance of properly assessing the decision to award the contract when they didn't even know of the existence of, let alone see, the key document that influenced the decision.

"This completely undermines the democratic process and I believe it contravenes the constitution. I have asked the Head of Law to revoke the conclusions of the cabinet scrutiny meeting of April 19.

Labour's George Nobbs, who has also complained to the council's monitoring officer and standards committee, about the issue, said the move was "cynical and contemptuous' and said it proved that the cabinet meeting was pre-scripted.

"I am asking that an investigation now takes place about any potential breaches of the council's constitution, its code of conduct, and of the law," Mr Nobbs said. I am also concerned about whether or not senior offices took part in the cabinet meeting despite knowing of this pre-determined decision.

"My call-in was based on the fact that the cabinet meeting was a charade and pre-ordained and this proves it was," Mr Nobbs said.

Mr Murphy said the briefing had been prepared following a request from the group and other parties were also entitled to seek information from officers.

"This is the biggest public sector investment in years," he said. "Anybody who thinks we do not discuss these things must be living on another planet. People will have their views, but we make our mind up on the day, otherwise what's the point of having a cabinet meeting? This was a private meeting of a political party which is outside of the constitution of the county council.

"If the Green Party wants a group meeting and they want information from officers, they are big boys and girls, they can ask the officers for it."