Chancellor John Healey has ordered an inquiry into the suspected leak of Budget bank-tax plans after details appeared before his statement to Parliament, prompting concerns that investors could have gained an early trading advantage.

James Bowler, the Treasury’s Permanent Secretary, confirmed to a parliamentary committee that the Government Security Group was investigating the potential disclosure of measures that may have moved financial markets. The committee has also urged the Financial Conduct Authority to examine the apparent early release.

The inquiry was expected to focus on a Bloomberg report published at 10.45am on Budget day. It said the increase in bank taxes would be temporary and less severe than lenders had feared. Healey subsequently announced that the bank corporation tax surcharge would rise from 3 per cent to 6 per cent for 15 months from January 2027, helping to fund measures intended to ease cost-of-living pressures.

The distinction between a tax increase and a smaller-than-expected increase matters to investors: advance knowledge of the latter could influence trading decisions even though banks would still face a higher charge. Committee chair Dame Meg Hillier said bank share prices appeared to react to the report before the Budget was presented. That concern does not establish who disclosed the information or whether anyone profited.

In a letter to the committee, Bowler said he had been assured that no banks, representatives or trade bodies had been briefed on the surcharge changes before the Chancellor’s announcement. That assurance leaves unresolved how the details reached newsrooms.

Hillier, the MP for Hackney South and Shoreditch, questioned whether the investigation would identify anyone responsible, citing what she described as the poor record of previous government leak inquiries.

“We cannot continue to accept this doom-loop of dead-end investigations in which everybody in the Treasury throws their hands up in despair, but nobody is ever found to have done anything wrong.”

She said she would press Healey for answers at his scheduled appearance before the committee on Wednesday afternoon. Alongside the search for the source, the unresolved issue is whether early disclosure affected trading before Parliament heard the policy.