Francis Maude, Shadow Secretary of State for National Heritage, launched the attack after a private meeting with Mr Mandelson last week, at which the minister said that funding for the Dome is on a "knife-edge", and appealed for more Tory support.
The admission of the weakness of sponsorship, which Mr Maude confirmed last night, contrasts with public statements from the Government on the health of the scheme.
Mr Maude, who also had a tour of the Dome's site two weeks ago, said: "We shall continue to take an independent view, while offering support in principle. The main problem in the way of this being a bi-partisan event is the way in which Mr Mandelson has hijacked it.
"It is a controversial project because he is the most controversial minister in the Government: half the press is out to get him and three-quarters of his backbenchers are also opposed."
The continuing controversy threatened to overshadow Government plans to highlight advancements in the millennium scheme this week.
Stansted, London's third airport, could become "Millennium International" as part of the celebrations for the year 2000.
Internal documents at the British Airports Authority show that Stansted will be titled "The Millennium Gateway" and will be "a focal point for additional air travel/charters etc to the Dome". Industry sources confirmed that BAA is also considering a name change for Stansted to "Millennium International".
BAA's pounds 3m support package will include backing for a project within the Dome and retitling the National Youth Games (to be run in partnership with the English Sports Council) the "Millennium Youth Games". The authority believes the Dome could produce more air traffic and raise awareness of Stansted.
It has also emerged that the Dome being erected in Greenwich, London, may be moved when the exhibition ends.
This week the Prime Minister will launch a presentation of the Dome's contents, announcing a pounds 12m sponsorship from British Telecom. Mr Blair will give the pounds 758m project his personal backing, declaring: "The time for carping is over and the time to support a great national endeavour has arrived."Reuse content