The Tory benches in the Commons could soon be losing one of their most recognisable occupants if voters hand the party the historic victory it is hoping for in today's Crewe and Nantwich by-election.

Boris Johnson, the new Mayor of London, promised to give up his Henley seat during the campaign to replace Ken Livingstone at City Hall, although he had not been expected to do so until the autumn, after the Parliamentary recess.

Party sources say, however, that David Cameron, the Tory leader, is considering a mid-summer poll to capitalise on a victory in Crewe, which would be the party's first by-election gain for 30 years.

The late Gwyneth Dunwoody held the Crewe seat for Labour with a majority of 7,000 at the last election, a margin which should be safe enough to allow her daughter, Tamsin, to succeed her on the benches at Westminster.

But after a botched class-based campaign by Labour and amid widespread voter disaffection with Gordon Brown, the Tories are now widely expected to overturn that majority. Expectations are so high that a panel of experts appointed by the PoliticsHome.com website has set the bar for a “really good result” for the Tories at a majority of 4,000. A small win - something which the party could hardly dream of even a few weeks ago - would be just an “OK result”.

In deciding the date of the Henley by-election, Conservative Central Office will also be keen to steal a march on the Liberal Democrats, who run second in the Oxfordshire constituency and have already appointed their candidate for the battle ahead.

If Mr Cameron decides to go for an early poll, the Tories would move the by-election writ - formally setting the machinery in motion - the week after next, setting the ground for voters to go to the polls on either June 26 or July 3. Mr Johnson would then be forced to give up the seat he won in 2001 and focus on his mayoral duties and journalism.

Officially, the Conservatives are not saying when the Henley by-election will be called.

Whatever the margin, a victory in Crewe would be a political milestone for the Tories.

Labour has held Crewe since the Second World War, and it is 165th on the Conservatives’ list of target seats.

It would be the party’s first by-election gain from another party - the now defunct SDP in Mitcham and Morden in 1982 and from Labour, Ilford North, in 1978 - a year before Margaret Thatcher ousted James Callaghan from No 10.

The Tories need a swing of 8.2 per cent to win Crewe and Nantwich - greater than the 6.9 per cent swing Mr Cameron needs to win an overall Commons majority at the next general election.

Apart from providing a psychological boost, victory in Crewe would cap a remarkable turnaround in Mr Cameron’s political fortunes.

Less than a year ago, after a party row over grammar schools and a mistimed visit to Rwanda during the summer floods, he was trailing in the polls as Gordon Brown replaced Tony Blair as Prime Minister on a wave of popular enthusiasm.

Earlier this month, Labour suffered its worst electoral performance for at least 40 years in the English and Welsh local elections, pushed into third place behind the Liberal Democrats, while the Conservatives also took control of London, through Mr Johnson.

The nightmare scenario for Labour is that they could be pushed into third place in Crewe and Nantwich behind the Liberal Democrats.

The recriminations have already started within the Labour Party, with some MPs admitting that the attempt to brand the Conservative candidate, Edward Timpson, a “Tory toff”, had backfired.

Mr Brown will seek to blame the expected defeat on mid-term discontent, with voters angry over rising food and fuel prices, and the 10p tax debacle.

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