Training providers are warning that a new, single literacy and numeracy programme to replace key skills and Skills for Life is causing success rates to plummet.
They argue that a system designed around school pupils does not meet the needs of apprentices and adults, and puts rising success rates and the achievements of Skills for Life at risk.
A spokesman for the Association of Learning Providers (ALP) said it had been "inundated" with cases where success rates had dropped sharply. One provider which had 92 per cent of apprentices passing saw that drop to 43 per cent, he said.
The Functional Skills Support Programme proposes one system of literacy and numeracy education for children in schools, teenagers in apprenticeships and adults.
But training providers say apprentices are rejecting classroom-based approaches, arguing that key skills integrated into their practical work was more effective.
"The development of functional skills had been focused primarily on GCSEs and Diploma providers," the ALP spokesman said. "There was little evidence that employer-based delivery was understood."
Alan Tuckett, director of Niace, the adult education body, said there were similar problems for the millions of adults who used Skills for Life, particularly those with literacy and numeracy problems.
"From what we know, there is too big a jump to classroom-based function skills from how they are working now," he said. "The core of the best of Skills for Life is workplace-embedded learning.
"We have real worries about the people the Skills for Life agenda was set up to help. It's the only thing for beginners who need systematic investment. Taking it away will again make them invisible."
Skills for Life, which has cost pound;5 billion in England since 2001, has been one of the Government's clearest successes, providing 2.8 million adults with their first qualifications in literacy or numeracy, and passing the 2.25 million target two years early. But a report in 2003 revealed the scale of the task: 6.8 million adults have literacy levels below entry level 3, equivalent to the average for 11-year-olds. Less progress has been made on numeracy, after literacy was made the initial focus.
Criticisms included a Government study which found little evidence that it has improved adults' chances of finding work or getting a raise, despite offering personal benefits such as increased confidence.
Professor Alison Wolf, from King's College London, also found in a three- year study that it had little impact on workplace productivity, arguing that adults needed far more teaching than the typical 30 hours.
But defenders of the programme argue that it simply shows the difficulty and investment necessary to tackle a long-standing problem, not a flaw in the provision.
A spokesperson for the Department for Business, Innovation and Skills said functional skills would be rolled out nationally in September. It is undergoing a pilot scheme to test its effectiveness.
"The intention is to replace Skills for Life literacy and numeracy qualifications from September 2012, subject to the outcomes of the pilot," she said.