The admission came as the Greater London Labour Party's investigation into the leak was abruptly halted by the sudden resignation of James Hunt, the by-election candidate, hours before he was due to be questioned.
Larry Whitty, national party general secretary, who was due to interview Mr Hunt, last night left little doubt over where the finger of blame should be pointed. 'Mr Hunt's action must be left to speak for itself,' he said.
Steve Molyneaux, Mr Hunt's agent during the campaign, denied he was the leaker but has admitted responsibility and apologised for 'his part in the circumstances which culminated in publication of the figures'. No further action will be taken against him.
Labour reaffirmed its condemnation of Liberal Democrat tactics during the campaign, criticised by Lord Lester QC in a report as 'pandering to racism'.
But Labour critics claimed the unauthorised returns played up the BNP's chances of winning and gave them credibility. Mr Whitty admitted last night: 'The unauthorised issuing of the figures did damage Labour's campaign.' The BNP captured the seat from Labour by 0.1 per cent, a margin of seven votes.
Mr Whitty's inquiry followed a highly secret investigation by the Bow and Poplar Constituency Labour Party. The Millwall branch was put under the supervision of the constituency party and the chairman and secretary resigned, but the inquiry failed to identify any culprits.
In a two-page resume of his inquiry, Mr Whitty said that the issuing of the fake returns had no form of 'go-ahead' from the constituency party, the by-election campaign committee or the regional party.
While it was impossible to assess the precise effect of the leak, 'it is clear that it had a negative effect on the morale of the party locally.'
A Liberal Democrat spokesman said: 'First they denied they were having an inquiry. Now they've admitted to one, but they still haven't found the answer to who leaked the faked canvass figures which led to the BNP victory.'
The Liberal Democrats have passed on two Labour leaflets, alleged by Lord Lester to be racist, and BNP literature, to Scotland Yard.Reuse content