London mayor Boris Johnson stood down yesterday as MP for Henley, triggering a by-election in the Oxfordshire constituency which will take place in three weeks time on Thursday, June 26.

The result from the safe Tory seat will be announced on the first anniversary of Gordon Brown taking office as Prime Minister. With a 12,793 margin of victory at the 2005 General Election, defeat for Tory candidate John Howell would be a virtually unthinkable upset for Mr Cameron.

That said, the Liberal Democrats are already campaigning hard in the seat, where they feel their candidate Stephen Kearney can make a dent on the Tory majority and their leader Nick Clegg is due to be in the constituency today. Also standing are Richard McKenzie for Labour, Mark Stevenson for the Greens and Chris Adams for Ukip. The by-election comes hot on the heels of Labour's defeat at Crewe and Nantwich on May 22.