Wednesday’s catastrophic pre-Budget leak of market-sensitive forecasts may have been caused by an “external person” who accessed a document via an obscure web link, rather than any deliberate act by OBR staff, the head of the Office for Budget Responsibility has admitted. In a frank interview on BBC Radio 4’s Today programme this morning, OBR chair Richard Hughes said he felt “personally mortified” by the incident that forced Chancellor Rachel Reeves to bring forward key announcements and triggered chaos in Parliament.

Mr Hughes told listeners: “I take full responsibility. On behalf of the OBR I regret the deep disruption that it caused to the Chancellor’s statement and parliamentary proceedings.” The leak saw the OBR’s full economic and fiscal outlook – normally published simultaneously with the Budget – appear online more than 40 minutes early, sparking instant ripples through financial markets.

Labour MPs privately described the episode as “humiliating” for Ms Reeves, who was seen checking her phone mid-session as the news broke, with Treasury ministers hurriedly passing notes along the frontbench. She was forced to rush out welfare reforms, energy bill cuts and £26bn in tax rises to limit the damage.

Mr Hughes revealed: “The documents weren’t published on our webpage itself” but were accessed through a link that “an external person” managed to find and exploit.

Pressed by presenter Amol Rajan on whether this pointed to a cybersecurity breach, Mr Hughes confirmed that former National Cyber Security Centre chief Professor Ciaran Martin has been drafted in to help lead an independent investigation.

Amol Rajan said: “Ciaran Martin is an expert in cyber security. There’s a hint there. I know you’re conducting an investigation. I know you don’t want to pre-empt it, but is the implication of what you’re saying that some sort of a link which you thought, which was available on your website, which you thought was, as it were, hidden and inaccessible, was possibly accessed by someone externally, ie, an external actor got hold of something on your website, rather than, you know, someone internally, pressing a button.”

Mr Hughes did not reject the suggestion, acknowledging: “We need to get to the bottom of what exactly happened.”

He promised the probe, overseen by the chair of the OBR’s oversight board, would be completed “quickly” and would report directly to the Treasury and the Treasury Select Committee. He pledged: “We will implement whatever changes are required to make sure it will never happen again.”

Mr Hughes repeatedly apologised, saying the independent watchdog “prides itself on our professionalism” and had “let people down”. He confirmed he had written personally to Rachel Reeves last night to apologise again.

Mr Hughes admitted: “This is very serious. When we make mistakes – whether they be mistakes in our forecast or mistakes in the way we run our systems and processes – we fess up to them, we find out what happened, and we make sure it doesn’t happen again.”

The admission that the breach came from outside the organisation will come as a relief to OBR staff, who had faced speculation of an internal error or even sabotage. But it raises fresh questions about the security of the complex technical arrangements used to share highly sensitive Budget material in the final hours before delivery.

Shadow Chancellor Mel Stride branded the leak “utterly outrageous” and suggested it “may constitute a criminal act”.

Tory leader Kemi Badenoch called the Budget “a total humiliation” for Ms Reeves.

A Downing Street spokesperson said Ms Reeves retains “full confidence” in Mr Hughes, despite the OBR boss vowing to step down if he lost the backing of Parliament or the Treasury.

Ms Reeves labelled the OBR’s blunder “a deeply disappointing and serious error”.

The OBR had already issued a public apology and described the early release as a “technical error”.

City analysts were quick to react to the leaked details, with bond markets twitching as the £26bn tax package – including employer National Insurance rises – emerged prematurely.

This is not the first Budget leak to plague Westminster – although the latest is unprecedented in terms of its scale. Past breaches have included accidental emails and misplaced USB drives, prompting renewed calls for urgent reform of the OBR’s processes. As the investigation unfolds, pressure mounts on the watchdog to restore trust in its handling of the UK’s most guarded economic secrets.