Boris Johnson has been accused of making misleading claims about the success of his key anti-youth crime project, despite warnings from officials they were inaccurate.
He told MPs in August that a special wing at Feltham Young Offenders' Institution was reducing reoffending from 80% to 19%.
But he has now admitted his officials told him of "caveats" around the data.
Mr Johnson insisted his error did not overshadow the success of the project.
He said it would reduce reoffending substantially in the end.
The Heron Wing was set up at Feltham in September 2009, a 30-bed unit for young offenders convicted mainly of less serious crimes and showing a "willingness to change".
Last November, on a visit with Prisons' Minister Nick Herbert, the mayor claimed it was cutting re-offending "dramatically" - from 80% to 14%.
Alarmed by these claims appearing in the media, Andrew Morley - the civil servant overseeing the project - wrote to City Hall officials in January this year, telling them they were not reliable and the effectiveness of the programme was "not yet known".
"This is just an indicator and should not be taken as the formal reconviction rate. As such it is not directly comparable to the national rate of reoffending for this age group," he wrote.
"This is primarily because the Ministry of Justice research standards follow young people for a period of 12 months in the community, whilst our tracking tracks everyone who has been released. Some of these boys would have been in the community for 12 months but some only a few weeks."
'Temporary and provisional'
Yet the mayor repeated the claims in the media, and then to MPs on the Home Affairs select committee in August.
"We cut reoffending rates in that wing from 80 per cent to 19 per cent. That is a model which should be replicated around the country," he said.
Last month the mayor told BBC London that no one at City Hall had told him that his claims were unreliable because the scheme had not yet been evaluated.
But on Wednesday he admitted officials had told him of "caveats" around the information about the unit, and its performance was yet to be confirmed.
He told the London Assembly: "If I erred, it was in not saying these figures are temporary and provisional and do not represent the final analysis. We will have to see how the Heron Wing fares in succeeding years and months."
Labour criticised the mayor for refusing to sign up to a national code on the public use of statistics.
Assembly member Joanne McCartney said: "Wilful misuse of stats in this way makes it impossible to know what's working and what isn't. Boris was told not to use these figures but did so, it appears, to claim a success without any evidence to back it up."
Last month Sir Michael Scholar, head of the UK Statistics Authority, raised concerns about the mayor's claims about reoffending at Feltham but Mr Johnson dismissed him as a "Labour stooge".
A spokesman for the UK Statistics Authority said: "The mayor is entitled to his views. The independence of the Statistics Authority and its chairman are evident from our published correspondence and reports."