An “external person” may have been able to access the link to the Office for Budget Responsibility (OBR)'s fiscal outlook which was published prematurely before the Budget, the head of the watchdog has said.
The document revealed the contents of Rachel Reeves' Budget and was accidentally published shortly before midday on Wednesday, half an hour before the Chancellor announced the measures.
In an interview with BBC Radio 4's Today programme on Thursday morning, OBR chairman Richard Hughes said of the investigation into the incident: “Well, the documents weren't published on our webpage itself.
“It appears there was a link that someone was able to access - an external person.
“We need to get to the bottom of what exactly happened. We're going to do a full investigation.
“There'll be a full report to Parliament.
“We're going to do that work quickly so people can have assurance in our systems and that can be restored.”
Mr Hughes told the programme the investigation would include expert insight from Professor Ciaran Martin, the former head of the National Cyber Security Centre, and would report to the Treasury and the Treasury Committee.
The OBR previously said a “technical error” within the organisation was to blame for the early release and apologised.
On Thursday, Rachel Reeves said she retains confidence in Mr Hughes despite the leak, which she said “must never happen again”.
The Chancellor told Sky News: “Richard Hughes wrote to me yesterday evening, apologising for their error. It was a serious error, a serious breach.
“They have announced an investigation which will report to me very quickly.
“But I do have confidence in Richard and the OBR. They do important work. But what happened yesterday, it did let me down, and it shouldn't have happened, and it must never happen again.”
Ms Reeves previously said she had found out about the early release while she was in the House of Commons preparing to deliver the Budget.
The OBR's report, which is usually released after a chancellor has delivered their Budget statement to the Commons, included growth forecasts, the extent of tax rises and the decision to lift the two-child cap on benefits.
