Peter Mandelson has defended his appointment as Britain's next European commissioner and said he will use the post to pursue economic reform.
Mr Mandelson said he would bring conviction to the commission job
The ex-cabinet minister, who is backed by Tony Blair, argued he will be able to raise Europe's profile in Britain.
He said if "fair-minded" people gave him a chance in the role, he was "determined not to let them down".
But the Tories say they believe he is too pro-European, while some Labour MPs criticised the move as "cronyism".
Mr Mandelson, who has twice resigned from the cabinet in controversial circumstances, refused to be drawn on whether he had discussed a further cabinet post with Mr Blair.
The appointment will trigger a Westminster by-election in Mr Mandelson's Hartlepool seat, which despite a 15,000 Labour majority is already being talked of as a winnable target by the Liberal Democrats and UKIP.
Mr Mandelson admitted he would miss being an MP and regretted having to leave his constituency in the autumn.
"To have that link with people is very, very important and I feel very sorry to give it up," he said.
Mr Mandelson told BBC News 24 he would bring "conviction" and a "commitment to reform" to the commission job.
He said: "I want to make sure the commission is properly pursuing economic reform and jobs but also projecting Europe's strength in the world, both to protect us from terrorism and wider security threats and to do good and improve the world."
MANDELSON'S NEW JOB
He will be the only UK politician on Europe's main decision-making body
As a commissioner he will help draft proposals for new EU laws
Commissioners also make sure EU laws are enacted
Former commissioners include Leon Brittan and Roy Jenkins
The 25 Commissioners earn £145,000 yearly salaries
They also get allowances for staying out of town
Mr Mandelson said Britain's influence in Europe had increased in recent years but he needed to raise Europe's stock with the British people.
But Tory co-party chairman Liam Fox said Mr Mandelson would not stand up for the UK in Europe as he was too pro-European.
Dr Fox also questioned whether "someone with his track record" could be trusted in such an important role.
Mr Mandelson defended his past, saying his exit from the cabinet had been for "bogus reasons" which had proved "wrongly founded and unfair".
"I hope people will be fair-minded, I hope people will feel that I'm going to put the best foot forward and give my all," he said.
"If people want to give me that break and that fairness, I'm determined not to let them down."
The prime minister said he had chosen Mr Mandelson because he was simply the "best person for the job".
"He has got the right skills, the talent, ability and contacts in Europe," he said.
1979-82 Lambeth councillor
1982-85 producer at London Weekend Television
Late 1980s Director of Labour Party Campaigns and Communications
1992 Became MP for Hartlepool
1994 supported Blair's leadership bid
1997 Ran Labour's election campaign
1997 Named Cabinet Minister Without Portfolio
1998 Secretary of State for Trade and Industry
1998 Resigned over Geoffrey Robinson house loan affair
1999 Returned as Secretary of State for Northern Ireland
2001 Resigned over Hinduja passports affair
"Europe is Britain's biggest economic market. It has expanded to 25 countries and this is a really critical appointment and we have got to get the best person and I think he is the best person for our country."
Mr Mandelson's appointment will have to be ratified by MEPs, but they can only veto it by rejecting the European Commission as a whole.
The MP is reported to have wanted to return to the cabinet for a third time - but the idea was dropped after fierce opposition from some senior ministers, including John Prescott.