Councillors who voted on the controversial Javelin Park Incinerator in 2015 without being told that the cost of the project had risen by a third are demanding an independent inquiry.

In an open letter to Gloucestershire County Council’s chief executive the politicians say that the £600m increase in the cost of the project, which were not made public, means that the whole project should be look at by someone who has not been involved in the contract so far.

Ten people have signed their name at the bottom of the letter, which accuses county council cabinet members of “misleading the public” by failing to reveal that the cost of the project had risen by 30 per cent.

In fact signatories including MP David Drew, Green Party county council leader Rachel Smith, and Dr Simon Pickering, chair of Stroud District Council’s environment committee, claim that throughout the process misleading figures have been used by council members.

Documents released by Gloucestershire County Council just before Christmas reveal that the lifetime cost of the Javelin Park Incinerator is set to top £600m, after the Cabinet secretly agreed to a 30 per cent increase in costs when renegotiating the contract in 2015/16.

The project, which was originally forecast to save the Council £1.2m over its first three years, is now set to cost at least £800,000 more than the alternatives over the same period.

Which councillors say will rob the budget of resources needed to pay for schools, social care and environmental improvement.

Cllr Simon Pickering has signed the letter calling for an inquiry

Cllr Rachel Smith, Green Party leader at the county council, said "Because information on the contract costs and structure has been withheld from councillors for so long, the council's own scrutiny processes have failed.

"We now need an independent inquiry to look at all the evidence fairly."

Another of the councillors who have added their name to the letter, Lib Dem leader Paul Hodgkinson, said: “Now that GCC have finally been forced into dropping their legal challenges to disclosure, we can see what they were trying to hide.

"A 30 per cent cost increase in 3 years is outrageous, and leaves big questions to be answered about whether this project can ever be value for money."

David Drew, MP for Stroud described the new information as 'scandalous'.

“What’s revealed about the nature of the contract by a campaigners' FOI request is truly scandalous,” he said.

"I am determined to pursue this matter at Parliamentary level and will be seeking an adjournment debate in House of Commons as a matter of urgency.”

Protestors plan to gather at the incinerator to demand answers

Green Party activist Tim Davies led the most recent FOI battle for documents to be disclosed said that the increased costs would lead to a 'tax on recycling'.

"It is the structure of this contract which is really outrageous - it places all the risk on the authority by guaranteeing payments of almost £190/tonne for 108,000 tonnes/year for the next 25 years.”

Last year, around 130,000 tonnes went to landfill: but with recent government policies on waste and with local action it is quite possible that the county will reduce its residual waste below the 108,000 tonne figure that the contract locks the council into.

"The less we burn, the most expensive the project. But even if the incinerator is used at full capacity, it still costs more than alternatives available today. The way the contract is structured creates a tax on recycling, and lumps costs on future generations,” Tim goes on to say.

“It looked bad when we saw the earlier contract at £150/tonne: but now at £190/tonne, even when electricity generation is taken into account, it is simply beyond belief that this deal could be in the public interest."

Local campaigners from NoMoInGlo and eXtinction Rebellion will gather outside the construction site of the Incinerator on Tuesday morning to draw attention to the cost increase, and to join the calls for an independent inquiry.

• A website will go live this morning with an FAQ page, as well as the full transcript of the letter, and details about the public can join the call for an independent inquiry.