As it was announced that the by election to replace Tony Blair as an MP will be held on the 19th or 26th July, it emerged that Scotland Yard detectives questioned him for a third time before he left office as part of the investigation into ‘cash for honours.’

In one of her last actions as Chief Whip, Jacqui Smith moved writs in the House of Commons to start the by election process.

Mr Brown appointed Ms Smith as Home Secretary today, the first woman to hold the office.

Voters in Ealing Southall will go to the polls on the same day to elect a new MP to replace Piara Khabra, who died last week.

Political insiders expect that gay candidates or those from ethnic minorities will be chosen to fight the vacant seats.

The by elections will be the first electoral test of Gordon Brown’s government.

A spokesman at Scotland Yard told the BBC that the Metropolitan Police was “not prepared to discuss” the inquiry.

Mr Blair has been questioned three times, and became the first British Prime Minister to be questioned as part of a criminal investigation.

136 other people, including several of Mr Blair’s closest aides, have been interviewed. Fundraiser Lord Levy and former Downing St staff member Ruth Turner were arrested by investigating officers.

At the 2005 general election Mr Blair had a majority of 18,449 over the Conservative candidate, while Mr Khabra had 11,440 majority over the Liberal Democrats.