Update: The UK Statistics Authority Chairman has now written to the Home Affairs Select Committee about the Mayor's figures. See the update below this piece.
Update 2: Chairman of the Home Affairs select committee Keith Vaz has released a statement to express his concern over the "allegations that the mayor of London may have misled the committee." The Labour MP says that he has written to Mr Johnson "requesting confirmation of the figures and an explanation."
After this summer's riots, the issue of youth crime is bound to be a political hot potato as campaigning increases ahead of next year's Mayoral election in London.
Current incumbent Boris Johnson is understandably keen to keep the focus on the progress he has made in tackling the issue in his three years at the helm.
When the Home Affairs select committee investigated the riots last month, Mr Johnson pointed to the success that one young offenders institute in Feltham has had in reducing reoffending.
He said: "the Heron wing at Feltham is so important... We cut reoffending rates in that wing from 80 per cent to 19 per cent. That is a model that I think should be replicated around the country."
Yesterday, the BBC cast further doubt over these figures.
Reoffending rates are calculated by measuring the proportion of offenders leaving custody that go on to be convicted of another crime within a year of their release. Because of the time it takes for offences to be processed by the courts, it often takes over 18 months to properly record the reoffending rate.
The Heron Unit at Feltham is a reasonably new initiative, opening in September 2009. However the figures being quoted by Mr Johnson have been around for over a year too soon for full reoffending rates to be available, as Straight Statistics noted at the time. Official reoffending rates are expected to be published in the spring.
The 19 per cent figure quoted by the London Mayor was based on "anecdotal evidence" compiled from recently released offenders, not all of whom had been out of custody for a full year. It was therefore not comparable with the national reoffending rate' of 80 per cent given by Mr Johnson.
However even the 80 per cent rate itself is out of date and has been contested by the Ministry of Justice, who pointed out that for those who had already previously been in custody, the rate was 73 per cent.
For offenders who had never been locked up before, the reoffending rate was closer to 60 per cent. Most of the offenders chosen to pilot the Heron Unit fall within this category, which may make it the better point of comparison.
According to the BBC, the current estimated reoffending rate for those released from Feltham is "at least 40 per cent". We have contacted the London Criminal Justice Partnership who are overseeing the initiative to confirm this, and they haven't yet done so.
If this estimate is accurate, then it is perfectly possible that the official statistics next year may show, as the BBC suggests, that "reoffending rates at Feltham could end up close to - or even more than [the national] figure."
It has to be said that Boris Johnson has form when it comes to prematurely using crime statistics. Back in March we referred the Mayor to the UK statistics watchdog after he used transport crime figures before they had been officially released, meaning that voters couldn't check the data for themselves.
Unfortunately, the Mayor doesn't fall under the Statistics Authority's remit, and while the national statistician has raised the anomaly as part of the review of crime statistics, both Mr Johnson and the Home Secretary have refused to budge.
While this may mean that we still lack a proper channel to redress inaccuracies such as these, Full Fact will certainly be on hand to ensure that they don't misinform voters ahead of the race for City hall in 2012.
Update: The Chairman of the UK Statistics Authority has now written to the Home Affairs Select Committee asking them to examine the figures used by the Mayor.
The letter, sent to Committee Chair Keith Vaz, urges MPs to look at the figures behind Mr Johnson's figures because at present they "do not appear to stand up to scrutiny."
The full text of the letter is here: