Pressure is mounting on Norfolk County Council to hold an external inquiry into the way the ruling Conservative administration carried out a decision to build an incinerator in King's Lynn amid claims that it breached council rules.

Last month the ruling Conservative group was accused of agreeing in secret to press ahead with the incinerator plans three days before the cabinet decision in March to award a contract to Anglo-US firm Cory-Wheelabrator after leaked internal Tory minutes of the group meeting showed 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'.

But there is rising anger that the minutes also suggest that the ruling Conservative group was made privy to officer advice which was not shared with other councillors - in breach of the council's constitution - after the minutes showed that deputy leader Ian Mackie gave the group a detailed briefing on the potential implications and commercial risks pressing ahead or not, or delaying the scheme, produced in conjunction with the council's heads of law, finance, and procurement, as well as solicitors Sharpe Pritchard.

Council leader Derrick Murphy has previously dismissed the allegations - insisting the briefing had been prepared following a request from the group and it was up to the other political parties to seek information from officers themselves.

But opposition councillors attempts to secure a copy of the briefing have so far been thwarted after they were told that it does not exist in paper form.

Green councillor Andrew Boswell has now written to council chief executive David White amid concerns the briefing has breached rules on officer impartiality and the council's constitution which states that they 'should not be used to pre-empt the decision-making process' while information given to a group leader, cabinet member, group spokesman, or committee chairman, will 'where possible, be shared with the relevant representatives of the other political parties'.

The external inquiry call, comes as the authority is still waiting on a government decision to award �169m of private finance credits to help build the scheme after Environment Secretary Caroline Spelman raised concerns about whether the scheme, which has sparked massive opposition, could demonstrate broad public support.

Dr Boswell has formally asked for a copy of the briefing and, in the absence of a paper document, copies of all emails, and background documents involved in producing it.

He is also pressing the chief executive to call in external experts to investigate and review all allegations about due process surrounding the incinerator contract.

'The council's image is badly tarnished by these events and it will continue to decline if these matters cannot be cleared up by such a review,' he said. 'There are three senior officers including the head of law, who were involved in producing the briefing, and it is inappropriate for such an investigation to be carried out by any person or body of Norfolk County Council.'

He also said the decision of the cabinet scrutiny committee to support the cabinet decision should be revoked because members were not privy to all the information available.

'This makes it clear that officers have a responsibility to make other councillors aware of the briefing and offered it to the opposition political groups,' he added. 'Further the failure to disclose the fact that advice had been given to the Conservative Party Group has resulted in inhibiting a full and complete scrutiny of the cabinet decision.'

Meanwhile Lib Dem leader and scrutiny committee chairman Paul Morse has joined the Labour group in calling for the council's standards committee to investigate, while anti-incinerator campaigner John Martin has formally lodged a complaint against those Tories who were also at the scrutiny meeting arguing that by taking part in the earlier vote they had failed to declared a 'prejudicial interest' and acted as 'judges in their own cause'.

Mr Morse said: 'The March cabinet meeting was so stage-managed that it was unbelievable, it was bad drama and theatre of the worst kind. If officers were aware of the decision that was made at the Conservative group meeting, then they were placed in an invidious and inappropriate position at cabinet three days later.'

A Norfolk County Council spokesman, said a response would be sent to the letters in due course.