“Proceeding in this way would cross a red line which would be impossible to defend as anything other than a political intervention,” he wrote.
The former chancellor said he was concerned the timing was designed to make the case that Budget tax rises were caused by a shortfall in the spending forecasts, that was not communicated to the OBR at the time.
Last week, Reeves said the claimed £22bn “hole” in the public finances was one of the reasons for tax rises and added the OBR would be publishing its review on “how that was allowed to happen”. The Treasury sees this as an important sub-plot to the main Budget narrative.
Hunt also hit out at the OBR for not asking for his views, or letting him see the report before publication.
On Sunday, the OBR responded to the former chancellor, saying the review was about the institutional relationship with the Treasury, not the conduct or decisions of ministers.
It also said that after advice from Cabinet Office, and concerns about market sensitivity, it was not “necessary or appropriate” to provide Hunt with advanced sight.
The OBR works closely with the Treasury, and its judgements on whether the chancellors’ plans are sound is important to financial investors and to Reeves, who has said she wants to borrow money to invest in big infrastructure projects.
Former prime minister Liz Truss and her chancellor Kwasi Kwarteng declined the offer of an OBR forecast ahead of their mini-Budget two years ago, which led to turmoil in the UK economy.
