Training company Learndirect has been branded by Ofsted as "inadequate" - the worst rating the watchdog can give.
Inspectors found not enough learners achieved the skills and qualifications to progress in work and education.
It also found teaching performance was not managed well by the firm, which has more than 70,000 trainees in England, Wales and Northern Ireland.
Learndirect, which had tried to block the report's release, said it did not accurately reflect its performance.
The company, which is one of the UK's largest adult training providers, had obtained an injunction against the publication of the report, but this was lifted by the High Court on Monday.
On Wednesday the government announced it would gradually wind down its contract with the firm, ending in July next year, over concerns about standards.
Ofsted rated the company as "requiring improvement" in five areas - including the quality of teaching, learning and assessment - and "inadequate" in two areas - its apprenticeships and outcomes for learners.
Its inspection in March judged Learndirect to be "inadequate" overall.
Ofsted found that the performance of Learndirect's subcontractors was not managed rigorously enough, with the result that apprentices who trained with them achieved "significantly less well" than those who were trained by Learndirect.
Too many 16 to 19-year-olds failed to complete their programmes and too few adults secured employment when they left Learndirect, Ofsted found.
The quality of teaching at the company, which employs more than 1,600 staff, was also criticised in the report.
Tutors and assessors failed to develop learners' and apprentices' English and mathematical skills well enough, the report said.
It also found they failed to use the assessment results of learners' prior skills to plan learning effectively.
The teachers did not provide helpful feedback at reviews to enable learners to improve their work and skills, Ofsted said.
However, tutors were judged to have provided good support for adult learners.
Ofsted also said new senior management had begun to tackle weaknesses and there were "early signs of improvement".
Learndirect said it was "financially stable" and would continue to meet its contractual obligations and the needs of learners.
The company said it had made improvements since the inspection and had rising learner satisfaction scores and success rates.
A spokesman added: "We maintain that the process behind Ofsted's report did not provide an accurate reflection of the current quality of Learndirect Limited's training and performance due to the unrepresentative sample size and the use of legacy data."