* Reporter at London newspaper tweets front-page budget details
* London Standard apologises, editor says "devastated" that embargo breached
* Opposition lawmakers call for investigation
* Treasury official says ministry to look into what had happened
LONDON, March 20 (Reuters) - Details of Britain's market moving budget were published on the Internet by a newspaper reporter 15 minutes before the finance minister stood up to give his speech on Wednesday, prompting calls for an investigation.
A copy of the front page of the London Evening Standard, containing details of economic forecasts, tax changes and borrowing, was published on Twitter well before George Osborne rose to his feet.
Some opposition lawmakers waved copies of the front page, which had been compiled with embargoed details of the speech, at Osborne while he spoke in the lower chamber of parliament, the House of Commons.
"He almost needn't have bothered coming to the House because the whole budget, including the market sensitive forecasts, were in the Standard before he rose to his feet," Ed Miliband, leader of the opposition Labour party, told parliament.
"I'm sure he'll investigate and report back to the House," Miliband said.
The leak electrified an uproarious session in parliament. At one point Deputy Speaker Lindsay Hoyle reprimanded Labour's finance spokesman, Ed Balls, for brandishing a copy of the Evening Standard front page at Osborne.
A Treasury official who spoke on condition that his name was not published said the ministry would look into how the leak had happened.
The newspaper's editor, Sarah Sands, apologised and said the paper's journalists were "devastated" that an embargo - in which something is given out under condition that it not be released until a set time - had been breached.
"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," Sands said.
Sands told the BBC that a young journalist had tweeted a copy of the front page.
"It happened, they've apologised. What can you do?" a Conservative Party source said.
The budget is supposed to be kept secret until the chancellor of the exchequer, as the finance minister is known in Britain, briefs parliament on its contents.
In 1947, Labour finance minister Hugh Dalton resigned after divulging details of his budget to a newspaper journalist before his statement to parliament.