Staff producing the short-list for the Radio 4 competition said they received an anonymous memo circulated to every member of staff in the party urging them to vote for Mr Blair. The note was sent by the Audience Participation Unit, a new unit encouraging members to write to newspapers and to attend political television shows, pushing the party line.
It told recipients they could fax nominations but it would be "preferable" to use a machine which did not identify the sender with Labour. Its purpose was to "avoid a repeat of last year, when John Major won" - in fact, the Prime Minister came second - by getting supporters to nominate Mr Blair.
A BBC statement last night said: "We deeply deprecate any attempt to interfere with ...a spontaneous opportunity for the programme's listeners to express their point of view. We have concluded that the poll should be closed forthwith."
In the event, Mr Blair failed to gain enough votes to make the short- list of six nominations when the deadline was brought forward 24 hours, from midday today.
A Labour spokesman said he did not believe anything improper had taken place; the memo merely "reminded" staff the poll was taking place. "A note was sent out to all members of Labour Party staff, who are undoubtedly Today listeners, urging them to participate in the ...poll. It was sent out by a unit whose primary tasks are to remind party activists and members of polls which are taking place nationally or locally, and to encourage people to apply to be in the audience of political programmes."
Asked about the letter last night, Mr Blair said: "As far as I am aware, it was done by an official acting without any authority. It is not something I would have sanctioned."
Brian Mawhinney, the Conservative Party chairman, said: "The Labour Party has been caught red-handed trying to cheat."
Yesterday's discovery is at least the second time the Today poll has been sabotaged this decade. In 1990, when Baroness Thatcher and Michael Heseltine won, a prominent Hindu politician was disqualified after a number of his British supporters tried to rig the voting. Lal Krishan Advani, leader of the Bharatiya Janata party, was removed from the short-list after BBC employees noticed a number of votes had been cast in the same hand writing.
Yesterday a former Conservative agent said Welsh Tories had mounted a similar attempt to boost Mr Major's rating in the poll two years ago. Elwyn Jones told the BBC: "It was definitely stated in the letter that this poll was taking place and our members should be urged to telephone the Today programme with their opinion that John Major should be Personality of the Year. I do remember that quite distinctly."
A Labour spokesman said: "It is typical of Brian Mawhinney ... that they go around throwing accusations at their political opponents and are immediately exposed for doing the exactly the same thing themselves." The row follows a second attempt to rig this year's BBC Sports Personality of the Year. Earlier in the week, organisers disqualified the footballer Justin Fashanu after being bombarded with e-mail votes for the player. They had to purge the system of a second intrusion, when the message "Let's get Damon Hill disqualified" appeared regularly on Internet soccer pages.Reuse content