A £100 million penalty will have to be paid if the Javelin Park incinerator contract with developers Urbaser Balfour Beatty is cancelled.

The figure was revealed after more than 400 protesters turned out in Stroud on Saturday to march in protest against the decision. But campaigners say the long-term savings to be had by cancelling the contract will far outweigh the initial compensation.

In 2013 there was a public outcry when Gloucestershire County Council signed a £500 million contract with developers Urbaser Balfour Beatty.

And last year the council’s own planning committee unanimously refused planning permission, but this went to appeal and was given the green light last week.

Now the Labour Party is calling for an extraordinary meeting to demand that the Tories cancel the contract.

Labour councillors and parliamentary candidate David Drew jointly announced that they want to hold a vote to cancel the council’s contract with the builder.

Although the Liberal Democrat party has condemned the decision by Eric Pickles to go ahead with the incinerator, leader of the Lib Dems Cllr Jeremy Hilton said: “We have seen the figures for exiting the waste incinerator contract with UBB and it is astronomical. The figures quoted are between £60m and £100m, which is far greater than the £33 million quoted for cancelling a similar contract in Norfolk.

“Labour has acted irresponsibly in calling to cancel this contract before knowing these costs. The Conservatives were reckless in signing such a rigid contract before the planning committee had even considered the application.

“The Liberal Democrats have always been opposed to the building of an incinerator and have continually pushed for alternative technologies such as Mechanical Biological treatment (MBT).

“Both Labour and the Conservatives should be ashamed of themselves in allowing this diabolical situation to develop in a ‘damned if you do, damned if you don’t’ position.”

The Lib Dems are now examining the numbers in detail and will be seeking legal advice.

Meanwhile, although Mr Pickles gave UBB the green light last week, he has since admitted he had no knowledge of the decision as it had been made by junior ministers.

Speaking to BBC Gloucestershire, Mr Pickles said: “Everything comes out in my name but it’s all decided by junior ministers. I have no involvement in that particular decision.”

Despite this Mr Pickles affirmed that he was “confident” that the decision was “sensible” and would stand up in a judicial review.

He added: “We take that kind of decision very seriously. It’s not taken on a political basis; it’s taken on a basis of evidence.”

At the demonstration on Saturday, organised by protest group GlosVAIN, more than 400 people gathered outside the Subscription Rooms with placards, banners and shouting slogans and collecting signatures for a petition to put pressure on Shire Hall to back out of the contract.

Commenting on the turnout in Stroud on Saturday, Jojo Mehta, of Toxin Free Community, said: “It was very clear from the several hundred-strong crowd at the demonstration called by GlosVAIN that there is no social licence for this project to go ahead."

“The emergency motion to the County Council proposed by David Drew to scrap the contract with the developer was very much supported by those gathered on Saturday.

A petition on the county council website currently stands at 2,000 and will force a debate if it reaches 5,000.