Former Northern Ireland secretary Mr Peter Mandelson is set to resign as a parliamentarian, clearing his way to become Britain's new European Commissioner, a government source said this evening.
British prime minister Mr Blair is expected to confirm his appointment to Brussels tomorrow, the source told Reuters news agency.
A fervent pro-European, Mr Mandelson has held two senior ministerial posts in Mr Blair's government but was forced out both times due to scandals.
Mr Mandelson's departure will force an election for his parliamentary seat in the northern England constituency of Hartlepool, raising the possibility of another embarrassing defeat for Mr Blair.
Mr Mandelson won it with a majority of nearly 15,000 at the 2001 general election but last week Labour lost one supposedly safe seat, and nearly another, in two by-election votes.
Mr Blair's spokesman said today the prime minister had to observe the courtesies of informing new Commission President Mr Jose Manuel Durao Barroso of his decision before going public and had not had a chance to speak to him.
The controversial Mr Mandelson has strong pro-European credentials and has long been a supporter of British membership of the EU single currency, the euro.