A bungling council which built a school “too small” may have to pay millions to fix the mistake.
Dargavel Primary School in Bishopton, Renfrewshire, Scotland, was made for 430 pupils but revised projections show it will need to hold 1,100 children.
Council chiefs have apologised for the "error" and now preparing to bring in portacabins next year to hold pupils at a cost £2 million.
Parents had apparently warned the new school, which opened in January, would not be big enough.
In a letter to parents, Steven Quinn, Director of Children's Services at Renfrewshire Council, told them they were sorry.
He said the calculation used at the time "identified the new school roll would not be more than 430 pupils".
Quinn said: “It became clear after a higher-than-expected number of admissions over this school year, the original projection would be significantly short of what will be required over future years.”
Quinn said their new "robust" projection "identifies the need to support a peak of around 1,100 pupils as Dargavel village grows over the next decade."
Architect Professor Alan Dunlop said the extension for a school to accommodate an additional 670 pupils could cost around £17million.
He added: “I'm bewildered how an ‘error’ can be made in the construction of a school.
“Design and construction of a school, or any building, including working through developing a brief and the various processes required and making and securing the necessary planning approval and building consent and warrants does not happen over night."
Katy Clark, the Scottish Labour MSP for West Scotland, said she was "flabbergasted" it had taken five years for the council to flag up the issue.
Conservative MSP, Russell Findlay, added: “For the council to get these basic sums so spectacularly wrong is embarrassing."
Quinn said a full review would be carried out into the circumstances, and the council would publish its findings.
The £18million school building opened this year after being delayed three times.
It is part of the privately funded Dargavel Village, a multimillion-pound development to build 4,000 new homes on the site of a former Royal Ordnance Factory by 2034.
As part of the agreement with the council, BAE systems paid for the new school.
But because they have built it to the specifications supplied to them by the council their obligations under what is known as the Section 75 agreement, are now discharged.
It means taxpayers will have to foot the bill to upscale the school.