Boris quits as MP to focus on Mayor job

Boris Johnson stepped down as MP for Henley yesterday leaving David Cameron facing another by-election test.

The constituency is considered a safe Tory seat – Mr Johnson won it in 2005 with a 12,793 majority when his predecessor Lord Heseltine retired.

But campaign organisers are mindful of the last time they fought a by-election in a 'safe' seat – Bromley and Chislehurst in 2006 – and their majority was slashed from 13,342 to 633.

Mr Cameron has told a private meeting that 'no seat is safe'.

A source at the meeting said: 'David knows that by-elections have a life of their own.'

The Tories' main challenger in the June 26 by-election will be the Liberal Democrats.

Labour is expected to do so badly it will lose its deposit.

Mr Johnson left the House of Commons yesterday by applying for the Crown job of Steward of the Manor of Northstead, immediately disqualifying him from being an MP.

He will now devote himself to being Mayor of London and, he said, making the capital a 'better, easier, more civil place'.

But his swansong to the Commons, which he called the 'great cockpit of our nation', was cut short by Speaker Michael Martin during Prime Minister's questions this afternoon.

Gordon Brown told him: 'I am sure the whole House will miss the contribution you make, not only the contributions in speech but in writing, which have actually been more significant over the last few years.'

The Henley by-election will be held on June 26 - the day before Mr Brown's first anniversary in No10.

Tory strategists are plotting a day of newspapers and TV bulletins plastered with stories of another victory for Mr Cameron alongside damning end of year reports on the Prime Minister.

A resounding triumph will also reinforce Mr Cameron's grip on the party - and turn up the heat on LibDem leader Nick Clegg.

In June 2006, the Tories were riven with internal rows over grammar schools and tax as they tried to hold Bromley and Chislehurst after the death of popular local MP Eric Forth.

Earlier this week, Mr Cameron, who is desperate to play down expectations, told a private gathering of staff at Conservative Campaign Headquarters during a pep talk that 'no seat is safe'.

The Tory campaign will be run by CCHQ by-election supremo Stephen Gilbert and fronted by shadow work and pensions secretary Chris Grayling.

Mr Cameron is also preparing to avenge an historic by-election defeat after LibDem MP Mark Oaten confirmed that he was considering leaving Parliament before the next General Election, expected in 2010.

His departure would trigger a by-election in Winchester, Hants, which the Tories lost at the 1997 General Election by just 2 votes, later increased to 7,476. Mr Oaten left frontline politics in January 2006 after revelations that he had paid male prostitutes for sex and is job-hunting in America.

He may be persuaded to stay on if the LibDems have a poor showing in Henley, or asked to go quickly if they do well.

Mr Johnson used his first news conference as London Mayor today to announce his retirement from the Commons to make the capital a 'better, easier, more civil place'.

Under a centuries-old procedure, Mr Johnson could not simply resign his seat - he had to disqualify himself from being an MP by applying for the sinecure job of steward of the Manor of Northstead.

The Henley poll will come a month after Labour's shock defeat at Crewe and Nantwich on May 22. But the party has trailed in third place in Henley since 1970 and the by-election offers little opportunity for Mr Brown to turn the electoral tide in his favour.

A Labour source said: 'We will be taking the opportunity to put David Cameron and the Tories on the spot over policy.'

If a party fails to poll at least five per cent of the overall vote loses its £500 deposit.