The Budget was thrown into chaos after an “unprecedented” leak of its contents hours before the Chancellor delivered her statement.
Rachel Reeves said the Office for Budget Responsibility's (OBR) accidentally releasing its forecasts before she revealed her Budget was "deeply disappointing".
The watchdog confirmed the Chancellor would be increasing taxes by £26billion in 2029-30 before MPs were told in the House of Commons.
Deputy speaker Nus Ghani also rebuked Treasury ministers for pre-Budget briefings to the press, which she said appeared to "have reached an unprecedented high".
Raising a point of order, shadow chancellor Sir Mel Stride told the Commons: "We have seen an unprecedented leak of the OBR's economic and fiscal outlook report before the Budget.
"This report contains market-sensitive information. It is utterly outrageous that this has happened and this leak may indeed constitute a criminal act."
The OBR’s leak revealed that forecast gross domestic product would grow by 1.5% this year, an increase from its earlier 1% forecast.
But it downgraded growth in 2026 from 1.9% to 1.4%, in 2027 from 1.8% to 1.5%, in 2028 from 1.7% to 1.5% and in 2029 from 1.8% to 1.5%.
The OBR document also confirmed Ms Reeves's Budget "raises taxes by amounts rising to £26 billion in 2029-30, through freezing personal tax thresholds and a host of smaller measures".
The freeze in tax thresholds will result in 780,000 more basic-rate, 920,000 more higher-rate, and 4,000 more additional-rate income tax payers in 2029/30, raking in about £8 billion for the Exchequer. The freeze will extend for three years to 2030/31.
Other personal tax changes include £4.7 billion through charging national insurance on salary-sacrificed pension contributions, and £2.1 billion through increasing tax rates on dividends, property and savings income by two percentage points.
The OBR said a "technical error" was to blame for the early release.
In a statement, it said: "A link to our economic and fiscal outlook document went live on our website too early this morning. It has been removed.
"We apologise for this technical error and have initiated an investigation into how this happened.
"We will be reporting to our oversight board, the Treasury and the Commons Treasury Committee on how this happened, and we will make sure this does not happen again.”
During Prime Minister’s Questions, Tory leader Kemi Badenoch said leaks and briefings out of Downing Street are "having real-world consequences" as she accused Labour insiders of "fighting like rats".
Deputy speaker Ms Ghani told the Commons: "For a number of weeks, and yet again yesterday, there have been extensive briefings to the media on the Government's fiscal policy and public finances.
"This disappointing trend in relation to Budget briefings has been growing for a number of years under successive governments, but appears to have reached an unprecedented high. "Weeks ago, we saw the Chancellor (Rachel Reeves) delivering a speech in Downing Street, setting a scene for the Budget, as well as specific policy announcements being briefed out to the media in advance of today's financial statements.
"And just a moment ago, it seems the OBR analysis has also appeared online. This all falls short of standards that the House expects.”
