Documents seen by Channel 4 news also suggest that Work Programme clients with health conditions and disabilities are "being seen less often and being offered less support than other groups".
Research by independent experts, Work Programme Evaluation: Interim meta-report, was ready to be published six months ago but ministers blocked its release, according to the programme.
It is reported to have found a quarter of providers thought it "very ineffective" while 22% believed it was "somewhat ineffective" and 10% thought it was "neither effective nor ineffective".
Under the Work Programme, providers from the voluntary, private and public sectors are paid according to results to get people into work, with extra incentives to support the hardest to help.
But the report states that there is "no clear relationship between an individual participant's Work Programme payment group and the nature and intensity of support that participant received, which suggested that the differential payment regime was unlikely to have effectively mitigated creaming and parking."
The Work Programme has come under attack since its launch in June 2011 and statistics last June showed that just 5% of the hardest to help people had found work.
Figures from Dr David Webster, of Glasgow University, suggested Work Programme contractors had been responsible for twice as many sanctions as they had produced 'job outcomes' - a job placements which lasts for a certain minimum period. Margaret Hodge, who chairs the Public Accounts Committee, said: "Officials assured us that the design of the programme was such that there was no way the private providers could ignore their (clients) needs. What the leaked report has shown us now is actually those needs have been ignored and those that need most help are not being given it."
A Department for Work and Pensions spokesman said: "The reality is that the Work Programme is working. Around half a million long-term unemployed claimants have started a job through the Work Programme.
"Previous schemes simply didn't do enough for disabled people or the long-term unemployed.
"We have taken action to drive performance up significantly and we are committed to making sure providers continue to improve the service they give to jobseekers."