Rishi Sunak’s government was accused of not following the law on revealing the scale of Britain’s public finances crisis earlier this year.
The bombshell claim was made by the independent spending watchdog, the Office for Budget Responsibility.
Giving evidence to the Commons Treasury committee on Tuesday, OBR chairman Richard Hughes said he thought there may have been a “misunderstanding” about the law.
Last week the OBR confirmed that the last government “did not provide” it with all available information at Chancellor Jeremy Hunt’s last Budget in March this year.
The watchdog said it had uncovered £9.5 billion in spending pressures that it was not made aware of ahead of the fiscal statement in March, and said that its judgement on spending would have been “materially different” had it had access to this information.
Mr Hughes told the MPs that he asked the Treasury earlier this year “what were you aware about in terms of departmental pressures at the time we were putting together the March Budget?”.
He added: “The short answer was there were about £9.5 billion worth of net pressure on departments’ budgets which they did not disclose to us (...) which under the law and under the act they should have done.”
Asked later in the session whether he thought the Treasury broke the law, Mr Hughes said: “They didn’t provide all of the information that we required to do our forecast.
“Now, there is always information that is relevant to the work that we do which we would like to have.
“In this case, this was a material amount of information which would have led to a materially different forecast had we had it.”
He said that there had been a “systemic failure” in the last forecast and potentially a “misunderstanding”.
He added that there may “have been a misunderstanding of how the law ought to be interpreted”.
He did not specifically point the finger of blame at Mr Hunt, Mr Sunak, any other minister or civil servants.
When asked about whether the Government thought former ministers had withheld information, Downing Street said on Tuesday afternoon that “civil servants advise and ministers decide”.
The Prime Minister’s official spokesman said: “At the Budget last week, the OBR published a review of of the issues surrounding the £22 billion black hole and how it came about and the Government has accepted all the recommendations made by the OBR, including recommendations around the previous processes being too informal.
“Now, obviously I can’t speak to previous administrations. But the general principle is that officials’ engagement with institutions like the OBR is obviously on the basis of ministerial policy positions.
“And you’ve got the Chancellor’s previous words that, in the end, civil servants advise and ministers decide.”
The OBR did not back up the allegation by Sir Keir Starmer’s government but it made clear it was unaware of nearly £10 billion of “spending pressures”.
New Chancellor Rachel Reeves announced £40 billion of tax rises in the Budget last week, £32 billion extra borrowing, to fund some £70 billion more spending including more than £20 billion more for the NHS.
