The political editor of the Evening Standard has been forced to apologise after the front page of Wednesday's issue was tweeted revealing key figures of the Budget, before George Osborne had even taken the stand.
Joe Murphy tweeted a two-part apology for the "very serious mistake".
The BBC's Nick Robinson called the leak "shocking".
After delivering the Budget, Labour leader, Ed Miliband suggested there was no point in Osborne attending the Commons "because the whole Budget, including the market-sensitive fiscal forecasts, were in the Standard".
He added: "To be fair to the Chancellor of the Exchequer, I'm sure he didn't intend the whole of the Budget to be in the Standard before he rose to his feet.
"I hope he will investigate and report back to the house."
Murphy appeared to sugget they would be making a formal apology to George Osborne and the House of Commons.
Evening Standard editor Sarah Sands said in a statement: "An investigation is immediately underway into how this front page was made public and the individual who Tweeted the page has been suspended while this takes place.
"We have immediately reviewed our procedures. We are devastated that an embargo was breached and offer our heartfelt apologies."
In 1947, Labour chancellor Hugh Dalton had to quit after a Budget was leaked before it was delivered in the Chamber.
The journalist responsible for the tweet has been suspended.