The dramatic move, seen by many as a political coup, came at the end of a week in which the proposed Millennium dome was saved from the axe by the Prime Minister.
Mr Heseltine, who is already a Millennium Commissioner, has been promised a "leading role" in the reshaped project.
More new faces will also be brought in under a restructuring of the board of the company running the Greenwich project, Millennium Central.
Mr Blair and Mr Mandelson, the minister now responsible for developments at Greenwich, have vetoed the idea that the dome, designed by Lord Rogers, be dismantled immediately after the Millennium show.
The latest ideas for the exhibition inside the dome are that it will be centred on the themes of time, how we live now and how we will be living in the next century.
Building work on Lord Rogers's centre-piece PVC-and- steel dome which will house the exhibition will begin next week as planned.
Mr Heseltine accepted his new role after just 10 minutes' consideration.
Mr Mandelson said yesterday: "Michael Heseltine worked tirelessly to make this project happen in the first place.
"He has a particular knowledge of this project and I think his involvement will give everyone, including business, extra confidence."
A Downing Street spokesman said: "The Prime Minister is delighted that Mr Heseltine is prepared to put politics aside and back this truly national project."
The invitation to Mr Heseltine was also seen as a smart gesture to the left of the Conservative party, following the election of William Hague last week. Downing Street sources said Mr Blair was "not tribal" about his politics and could even consider a role for the former chancellor, Kenneth Clarke, who retired to the backbenches after losing Thursday's final Tory leadership ballot.
Senior government sources insist that a realignment of politics remains possible.
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