Mandelson stepped down as an MP on Wednesday

The election was triggered by Peter Mandelson's appointment as Britain's new European Commissioner.

Former Cabinet minister Mr Mandelson formally stepped down as a member of Parliament on Wednesday.

The poll to choose his successor will be held on the last day of Labour's conference. The party is defending a 14,571 majority in the seat.

The formal writ for the by-election is expected to be moved in the House of Commons on Thursday.

Mr Mandelson has meanwhile used an arcane technical procedure to resign his seat.

He has been appointed the Steward and Bailiff of the Manor of Northstead.

Technically, MPs cannot resign their seats and have to disqualify themselves from being a member of the Commons by applying for an office of profit under the Crown.

The manor of Northstead is in fact under a lake near Scarborough and the post will give Mr Mandelson no income.

'Privilege'

Marking his exit from Parliament, Mr Mandelson said: "I would like to thank the people of Hartlepool for allowing me to serve as the town's MP for over a decade.

"It has been the biggest privilege of my life. I am very pleased the town's Labour Party has chosen an excellent local candidate to succeed me.

"If elected, he could not wish for better people to represent in the House of Commons."

Home Secretary David Blunkett was campaigning in the seat on Wednesday for Labour candidate Iain Wright.