The first thing that Winston Churchill did upon becoming a Cabinet minister in 1908 was to resign his seat in Parliament. He then proceeded to lose the resulting by-election. He had to scrabble around for a new seat before he could take office.

Churchill had not being acting on a whim. It was, in those days, a constitutional requirement that on the first occasion that you accepted a ministerial position you would offer your constituents the chance to deliver their verdict.

If Labour were to replace Gordon Brown as leader, there would be no constitutional requirement for it to ask anybody for permission. The convention that required Churchill to stand down was finally abolished in 1926 with the passage of the Re-Election of Ministers (Amendment) Act. And there are no other rules that prevent the majority party from behaving exactly how it wishes until its full parliamentary term is complete. Yet Labour MPs should be under no illusions. It would be absolutely unacceptable for the party to pick a new Prime Minister without seeking a fresh mandate reasonably soon after making the change.

Last year, when Gordon Brown moved in to 10 Downing Street, it was widely agreed that he did not need to call an election if he did not wish to do so. There were two reasons for this consensus. The first is that in the 2005 election it was well known that he was likely to take the place of Tony Blair at some point during the Parliament. Indeed, it became an election issue. In his memo to allies, which became public yesterday (see page 1), Mr Blair complains of the extent to which Mr Brown has presented himself as a change from the past. Yet during the last campaign the prospect of such a change was used as an inducement to wavering Labour voters.

The second reason for the broad agreement was the existence of precedents. In recent times both James Callaghan and John Major became Prime Minister and then remained in office into the fifth year of the Parliament. When the Conservatives challenged Mr Brown's legitimacy, Labour was able to point out that it was only acting as the Tories had acted.

If Mr Brown were now to be replaced by a new Prime Minister, the situation would be entirely different. The new Labour leader would be someone whose premiership had not been anticipated at all in 2005. And they would have been selected with a mandate to change the direction of the Government again. It is not reasonable that this mandate should come only from that odd collection of constituency representatives, union bosses and MPs that makes up Labour's electoral college. The public would feel entitled to have its say. And the public would be right.

Labour could not hide behind precedent either. Since the establishment of the modern franchise in 1867 there has only been one Government that has changed leaders twice without going to the country. Neville Chamberlain, whose premiership was sandwiched between those of Stanley Baldwin and Winston Churchill, resigned without facing voters at all. The circumstances were not, however, usual - the Parliament lasted ten years because of the war. Labour could hardly cite Chamberlain in its defence.

The political purpose of replacing Mr Brown would be for Labour to make another fresh start. If would be able to do so only if it asked for, and received, support from the voters. A new leader would not need to call an election instantly, but he or she should not tarry too long. If they did so, Labour would pay a heavy political price. Voters do not feel, to say the least, that allowing Gordon Brown to succeed without an election led to a sustained period of good goverment. Next time round, if there is to be a next time, Labour will find voters insistent.