They call for a moratorium on all private contracts until the centres have been properly evaluated and investigated.
The £5bn ISTC programme aims to provide extra capacity to the NHS and reduce waiting times for elective surgery.
The researchers have based their estimate on detail of a Scottish contract they obtained under the Freedom of Information Act.
They say that because the ISTC performed only 18% of the total annual contract value that Scottish health boards may have overpaid up to £3m in the first year of the contract. If the same findings apply in England, they estimate overpayment of £927m or almost two-thirds of the total first wave contracts.
- Were ISTCs a waste of money?
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