The London Evening Standard tweeted a picture of its front-page Budget splash before George Osborne spoke in Parliament this afternoon.
The newspaper tweeted the front page, headlined 'Things can only get bitter', around 20 minutes before the Chancellor of the Exchequer delivered his statement. The paper appeared to have advance knowledge of much of the contents of Chancellor George Osborne's speech.
Evening Standard political editor Joe Murphy tweeted shortly afterwards: "I wish to apologise for a very serious mistake by the @EveningStandard earlier which resulted in our front page being tweeted...
"We are so sorry to the House of Commons, to the Speaker and to the Chancellor for what happened. We shall be apologising to them."
Journalists are usually given no official advance knowledge of the contents of the Chancellor's speech.
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."
BBC business editor Robert Peston has quoted Sands telling him that the Budget leak is "an arrangement we have had with successive governments" and said it is because the paper goes to print before the Budget announcement.
She also reportedly told him it was "tweeted by a very young and inexperienced journalist".
While Chancellor George Osborne was on his feet delivering the Budget, the Labour front bench were closely studying photocopies of the Standard splash.
In 1947, Labour chancellor Hugh Dalton had to quit after his Budget was leaked before it was delivered in the Chamber.
The Standard's front page detailed several aspects of the Budget relating to duty changes, tax and macroeconomic figures.
Here are some Twitter reactions:
— Martin Belam (@MartinBelam) March 20, 2013
The Evening Standard looks like it has decided to leak every single key #Budget announcement
— Ian Jones (@msnnews_ian) March 20, 2013
No need to listen to George's speech at 12.30pm - appears the whole Budget is already in the London Evening Standard twitpic.com/ccwb9h
— Jack Blanchard (@JackBlanchardYP) March 20, 2013
Ed Miliband has front page of Evening Standard with Osborne's Budget spun by his team in advance. Trouble ahead
— Kevin Maguire (@Kevin_Maguire) March 20, 2013
Major breach that Evening Standard has accidentally leaked the Budget's contents. Lucky break for Ed Miliband, though.
— Stephanie Flanders (@BBCStephanie) March 20, 2013
Standby for row about Budget leak. Leak of London Evening Standard front page with the headlines twitpic.com/ccwb9h
— Nick Robinson (@bbcnickrobinson) March 20, 2013