More than 150,000 adults could be robbed of a "second chance" in education because of plans to make them pay the full cost of their courses, civil servants have warned.

At present, further education students over the age of 24 only have to meet half the cost, while under-19s are not charged. But from next August, the Government intends to compel all over-24s taking A-levels or access courses to pay the full amount, and is introducing loans up to the value of £3,700 to cover the cost. Only those seeking basic qualifications in literacy and numeracy will receive grants which they do not have to pay back.

The move is going ahead despite a warning from civil servants to ministers in an impact assessment that 45 per cent of those on such courses would be put off by a loans system.

If the assessment is right, almost 170,000 fewer students would take the courses, prompting closures and redundancies amongst staff, lecturers' leaders warned.

At present, there are about 376,000 people on the courses, one of the most popular being A-level business studies. The qualification has helped thousands of people – many of them women returning to work after having children – to start their own businesses.

Gordon Marsden, Labour's spokesman on further education and skills, has written to the Business Secretary, Vince Cable, demanding that the loans system is reassessed. "Adult further education is rightly viewed as a key lever of social mobility, by giving people who missed out first time around a second chance to gain qualifications," he said.

"Faced with the prospect of having to take out loans worth up to £4,000 a year, how many people from the most vulnerable and disadvantaged communities will feel able to take that bold step back into learning?"

Pam Tatlow, chief executive of the university think-tank million+, accused the Coalition of delivering a "double whammy" to mature students. "If they have to pay off their further education loads over a 30-year period, how many of them are going to go on and take out another 30-year loan to cover the cost of a higher education course?" she asked.

Union leaders warned the plan was bound to lead to significant job losses as well as destroying the hopes of thousands of adults.

Regulations to introduce the new loans system are expected to be laid before the House of Commons ahead of the summer recess. They will be managed by the Student Loans Company, which already deals with undergraduate payments.

The impact assessment says: "The central estimate is based on an assessment of research which suggests that around 55 per cent of the learners who would have been supported... would go ahead with such a system of loans."

A spokesman for the Department for Business, Innovation and Skills said there would be a 40-day period after the regulations had been tabled for any MP to table a "negative resolution" against them. "The introduction of loans is an important change for this sector," he said, "so we are working closely with colleges and training organisations to ensure they have the information they need to prepare. Take-up of loans and the impact on learners will be monitored closely."

The Association of Colleges said that although it had "serious concerns" about the loans, it feared any alternative scheme would mean worse cuts.

Late learners: From waiter to barrister

'The option has to be there for adults to return to learning'

Saiful Islam, 39, was working as a waiter in an Indian restaurant when he enrolled on an Access to Higher Education course in 1995. He now practices as a barrister for top chambers in London

"I left school at 16 without GCSEs. In my early twenties I was working as a waiter in an Indian restaurant. I had always regretted not studying and wanted a change of direction, so I set my sights on studying to qualify as a barrister and enrolled on an Access to Higher Education course at Barnfield College in Luton. I had to work at the restaurant full-time while studying, finishing at about 1am then going to college in the morning, but fortunately the course was inexpensive in those days. The course enabled me to get a place on a two-year, accelerated law degree at the University of Hertfordshire. By 2003 I had qualified as a solicitor and transferred to the bar two years later. Access courses are invaluable for people who want to improve their chances later in life. I was born in Bangladesh and when I look at poorer countries like it, I think it is a lack of these sorts of opportunities that holds them back. The option has to be there for adults to return to learning and it would be very unfortunate if it became more difficult to do."

'If the funding changed and I couldn't do another course I'd be gutted'

Margaret Green, 49, from Golcar, Huddersfield, left school at the age of 16. She worked in a carpet factory for 23 years and is now a Pharmacy Assistant at the Huddersfield Royal Infirmary. With funding from her local NHS trust, she completed a part-time Pharmacy Apprenticeship with Bradford College in March