Project Daedalus involved selecting 220 first-time offenders to serve their sentences in a separate wing of Feltham young offenders' institution, the Heron unit, to help them back into jobs or education.
The original draft report into the scheme, commissioned by the Greater London Authority (GLA), said the mayor's project suffered because City Hall's "payment-by-results" approach denied the scheme sufficient upfront funding, but this was removed before the final report was made public.
The mayor's office was not available for comment to the Guardian at the time of publication, but the BBC reported that it has refused to say whether it suggested rewrites to the report.
Young offenders selected to go into Heron unit were assigned "resettlement brokers" ? employed by the charity Rathbone ? to help them after their release, in a scheme that cost nearly £3m.
Half of the funding came from the European Social Fund and half from the mayor's London Development Agency (LDA), which insisted on the payment-by-results model ? the coalition government's favoured approache to tackling crime.
The independent evaluation of the scheme, conducted by market research company Ipsos Mori and published in March, reported that Rathbone had been paid according to its success in meeting a range of targets including the number of offenders helped into education, training or employment (ETE). Only one in six remained in ETE for six months ? a key target.
The official rate of reoffending among the 220 inmates who took part in the project will be released this summer.
The report authors found that Rathbone was not paid in advance and had to spend too much time on targets and the claims process for payment.
The original draft report said: "It was hard for them to invest the necessary funds upfront for some of the innovation.
"Indeed, a number of stakeholders commented on the fact that Rathbone is restricted to some extent by the fact that they are not paid upfront; it was argued that receiving a percentage of their payments in advance of meeting targets may make more sense."
The report's authors were told that payment-by-results models could dissuade charitable organisations from getting involved in future projects, a finding omitted from the final report.
In a single paragraph examining the funding process, the final published report mentions "frustrations" with the payment-by-results model.
It said: "Resettlement brokers said they would at times feel conflicted in their working practice; there was an uncertainty as to whether their key concern at any one time should be meeting targets, or meeting the needs of young people."