The Government is pouring billions into building new free schools in England while existing state schools are crumbling due to lack of repairs, the Whitehall spending watchdog has warned.
The National Audit Office (NAO) said £6.7 billion was needed just to bring all existing school buildings up to a "satisfactory" standard, with a further £7.1 billon required to restore them to a "good" condition.
But with ministers committed to creating 500 new free schools by 2020, the NAO said the Department for Education (DfE) was facing an estimated bill of £2.5 billion by 2022 simply to purchase the land needed to build them.
It said the DfE has already spent £863 million on land acquisitions for free schools over the last five years - in some cases paying "premium" prices because of a shortage of suitable sites.
While free schools were helping to meet the demand for additional school places in some areas, the NAO said that because local authorities did not control their numbers they were not necessarily "fully aligned" with their needs.
Some free schools were opening in areas where there were already plenty of places, creating "spare capacity" which could affect the future financial sustainability of other schools in the area, it said.
The DfE has estimated that of the 113,500 new places being opened in mainstream free schools between 2015 and 2021, 57,000 would create spare capacity in other nearby schools, potentially affecting their future funding.
Official data indicated creation of spare places in 52 free schools which opened in 2015 alone would have a "moderate or high impact" on the funding of 282 other schools.
At the same time, the NAO warned the condition of existing schools was worsening, with around 40% of the schools estate built between 1945 and 1976 coming up for replacement or major refurbishment.
As a result, the cost of restoring all schools to a satisfactory condition was expected to double over the course of the five years to 2020-21.
The chairman of the Commons Public Accounts Committee, Meg Hillier, said: "The DfE needs to spend at least £6.7 billion just to bring all school buildings up to a satisfactory state.
"But the department is choosing to open new free schools in areas which do not need them and are failing to fill places.
"This is taxpayers' money that could be used to fund much-needed improvements in thousands of existing school buildings."
The NAO said that the DfE was planning to replace more than 500 school buildings in the very worst condition by 2021.
However it warned that ministers faced "significant challenges" in providing the additional 420,000 school places needed by that date.
The chair of the Local Government Association's children and young people board, Richard Watts, said councils were "under extreme pressure to accommodate rapidly rising numbers of pupils looking for a place".
"No child should be without a place but councils fear that they will no longer be able to meet the rising costs for the creation of spaces, nor find the space for new classes, if they aren't given the money or powers to do so," said Cllr Watts.
"If the crisis is to be dealt with properly the Government must commit to funding the creation of school places and hand powers back to councils so that they can open new schools, for both primary and secondary-age pupils."
Shadow education secretary Angela Rayner said: "Hundreds of secondary schools in England are already full to capacity, while thousands more are crumbling and in desperate need of repair.
"Yet the Tories continue to waste millions opening free schools in areas that do not need them.
"The Government needs to concentrate on increasing capacity and improving schools in the areas that need it the most."
Liberal Democrat education spokesman John Pugh described the report as "a devastating indictment of Government financial mismanagement" and revealed a free schools movement which was "driven entirely by ideology and is directly against the educational interests of children and financial interests of the country".
A Department for Education spokesman said: "As the NAO acknowledges, we have made more school places available, and in the best schools. The free school programme is a vital part of this - more than three quarters of free schools have been approved in areas where there is already demand for new places and the vast majority are rated good or outstanding by Ofsted.
"The Government is making a huge investment in the school estate of £23 billion up to 2021, to create a further 600,000 new school places, deliver 500 new free schools, and rebuild and refurbish buildings at over 500 schools.
"But we want to go further. That's why we have set out plans to create more good school places, in more parts of the country, by scrapping the ban on new grammar schools, as well as harnessing the expertise and resources of our universities, and our independent and faith schools."