Half of all lecturers won new rights this week that could strike a blow at the increasing casualisation of college teaching.
New regulations offering hourly-paid lecturers and those on short-term contracts the same rights as full-time staff came into force on Monday.
More than half the lecturers in colleges are employed on temporary or hourly-paid contracts, according to the University and College Union.
Colleges rely on casual labour more than the building industry, the union claims, with only hotels and catering having more temporary staff.
The changes in the law mean that about 60,000 lecturers will be able to enjoy holiday pay and benefits equivalent to those of permanent staff if their contracts are renewed after four years' service.
In some parts of the FE system, the proportion of temporary workers is even higher. The union says that 90 per cent of lecturers in adult education could be affected.
Roger Kline, the union's head of equality and employment rights, said:
"There are vast numbers of further education staff who are on the contracts that will be affected by this: anyone on hourly-paid or fixed-term contracts; the essence is that they are temporary.
"After July 10, if they have four years' service, they will move to permanent contracts.
"Over the years, the sector has become very dependent on this kind of temporary contract. It's going to come to an end."
Breaks in employment over the summer and other holidays will not prevent a lecturer taking up the new rights. As well as paid holiday equivalent to permanent staff, lecturers will also accrue sick pay and be able to get incremental pay rises according to their length of service.
Mr Kline said he feared that colleges had not planned for the costs of the new rights, which could provoke disputes in September. "I'm not sure that employers have thought this through," he said. "But they are going to have to."
Colleges will not be able to make staff on temporary contracts redundant to prevent them benefitting from the new rights, he said, unless their employment was coming to an end and they were not going to be replaced.
Mr Kline said he also anticipated a struggle with colleges about what sort of permanent contract should be offered. While the union will argue for equality with the existing permanent staff, he said he feared some would be offered inferior "zero hours" contracts - where the college does not guarantee any work. He said: "What we are saying to the lecturers concerned is, assume you are moving to a permanent contract. Make sure it is not less favourable than your colleagues' contract. And, if anybody says you are not going to get a permanent contract, go to your union for advice."
The union will also be pressing for temporary lecturers to get their own offices, to improve their student support.
Others said students are likely to benefit from lecturers having increased security, especially in under-pressure areas such as adult education.
Alan Tuckett, director of the National Institute of Adult Continuing Education, said: "It creates a more suitable working environment for staff, who can spend less time anxious about holding on to their existing students for fear of losing their job, and worry more about moving people on.
"What learners need is the support of creative teachers who have got the time to do their job properly. If you get treated properly - and that means reasonable pay as well as access to training and development - then that can only be good for learners."