Already the Local Government Association has raised the alarm warning that it expects the cost of building extra teaching facilities will soar to £12bn, way ahead of the government’s commitment of £7.35bn for extra school places.

But cost consultant EC Harris is now warning the final bill will by 50% higher than that being reported by the LGA.

Marcus Fagent, Head of Education at EC Harris, said: “We have analysed our benchmarking data, drawn from our work on school projects with local authorities and the Department for Education, and we believe that a more accurate figure would be in excess of £18bn.

He warned that several factors were working together to drive up costs.

Fagent said a large proportion of the places, perhaps 40% will be achieved by expanding existing schools. This is more expensive to do, per child place created than building new schools,

He added that most of the ‘easy’ school expansions have already been done, and local authorities are now faced with addressing the projects that are more difficult and expensive because of the nature of the site.

Increasingly, the Government is having to buy the land to build new schools, paying market prices in urban areas where values are rising.

He said: “Cheap and easy temporary fixes are no longer available, and construction costs are rising.”

EC Harris argues part of the solution will be for local councils to drive forward design standardisation and seek to batch up work.

Councils will also need to drive value from scarce land resources.

“If public bodies are paying high prices for land they are going to have to consider creating mixed developments where housing and other commercial uses are built above and around schools to create value.

“This could extend to buying school space from investors in the same way as commercial property space. This might be a more flexible way of creating space and ultimately better value for the public purse.”