The Prime Minister will resign as MP for Sedgefield within hours of leaving Downing St, it has been reported.
He is expected to take up a new peace-keeping role in the Middle East on behalf of the international community almost immediately.
It was expected that Mr Blair, who has introduced a wide range of gay rights in his ten years in power, would not remain in the House of Commons after the next general election.
He has represented the Sedgefield constituency since 1983.
His immediate departure could mean that new Prime Minister Gordon Brown faces two by election tests within weeks of taking office.
Last week the Labour MP for Ealing Southall, Piara Khabra, died aged 82.
Mr Blair will make his final appearance at the dispatch box tomorrow at Prime Ministers Questions, and then will go to see the Queen and resign.
He is reported to have a meeting tomorrow afternoon with Labour party officials in his Durham constituency.
“If he gets a job on the worldwide stage, I think there will be a by-election,” a local activist told the BBC.
Mr Blair is expected to be confirmed as an peace envoy of the so-called ‘Mid-East quartet,’ namely the US, Russia, the UN and the EU, tomorrow.
Traditionally former Prime Ministers are offered a seat in the House of Lords once they leave the Commons.
It is not known if the title of Baron Blair of Sedgefield will be created for or accepted by the 54-year-old departing leader.