The chancellor's Budget decisions are supposed to be totally secret before his speech.
During former Chancellor Gordon Brown's time at Number 11, he installed a system of briefing for the press around his pre-Budget Statement.
There are also several famous cases where leaks were not quite so well managed:
In 1936 Jimmy Thomas, a Cabinet minister, was found guilty by a Tribunal of Inquiry of leaking Budget proposals to Sir Alfred Butt, Conservative Member of Parliament for Balham and Tooting.
It was also believed that Thomas had divulged Budget secrets to a friend and business associate, Alfred Cosher Bates for personal gain.
Thomas resigned from Baldwin's government, and then, shortly afterwards, resigned his Commons seat.
Following the 1947 Budget, Hugh Dalton resigned as Chancellor of the Exchequer after it was discovered that he had divulged plans for his Budget to a lobby journalist from The Star on the day of his statement.
The news appeared in the paper before Dalton left the Chamber. He was left with no alternative but to apologise to the House and resign.
Sir Gerald Nabarro
In the lead up to the budget in 1969, the Conservative MP Sir Gerald Nabarro's repeated claims that the car tax or vehicle excise duty was about to be raised in the Budget (from £25 to £35 per annum) prompted the prime minister to set up a Select Committee to investigate a possible breach of Budget secrecy.
Despite Sir Gerald's claims that civil servants and members of the public alike had informed him of the impending rise the committee found no evidence of a leak.
Even so, the publicity may have prevented car tax being raised that year.
Kenneth Clarke - 1996
Kenneth Clarke's final Budget in November 1996 was leaked almost in its entirety, on the eve of its presentation, to the Daily Mirror newspaper.
The Mirror refused to publish the contents, conscientiously returning it to the Treasury.