Academics are to boycott marking coursework from next week, potentially leaving some students unable to graduate later this year.
Exams and essay-marking could be hit by industrial action
The Association of University Teachers said its members could also refuse to do work associated with running exams unless pay and conditions improved.
The threat comes as university staff in Scotland hold a one-day strike as part of a UK-wide week of action.
The AUT says an offer of a 6.44% pay rise over two years is an "insult".
Its general secretary, Sally Hunt, said thousands of lectures had been cancelled and staff and students
had been out on hundreds of picket lines since Monday.
Members walked out in Wales on Monday, England on Tuesday, across the UK on Wednesday and Scotland on Thursday. A further protest in Northern Ireland is scheduled for Friday.
But the Universities and Colleges Employers Association (UCEA) says they have had little effect on academic life.
Its straw poll found 56% of targeted campuses had reported no noticeable impact.
The AUT has organised its action to coincide with a week of disruption by the National Union of Students, protesting against plans for annual tuition fees of up to £3,000.
The Higher Education Bill, which contains the proposal, was voted through by a majority of just five MPs last month.
Ms Hunt said AUT members would prefer not to refuse to mark their students' work and there was "still time" for the UCEA to come back to negotiate.
NUS president Mandy Telford said it was a "difficult decision" for lecturers, and students "understood why it has to happen".
She added: "NUS will continue to stand shoulder to shoulder in solidarity with our staff and lecturers up and down the country.
"It is a very difficult situation for students but they understand and totally support the AUT in the action they are taking."
Both groups have complained of a "marketisation" of higher education.
As part of the protest, lecturers will be "selling degrees" to students outside the Scotland Office in Edinburgh. Students will be paying with IOUs.
The AUT, which mainly represents staff at the older universities, established before 1992, estimates lecturers' wages have fallen by 40% compared with other workers' in two decades.
It opposes plans to end nationwide pay negotiations and change the grades of some staff.
Of the union's 47,000 members, 54.4% took part in a postal ballot on strike action. Of these, 66.6% were in favour.
Meanwhile some further education colleges also faced disruption, as members of the Natfhe union held a one-day strike over pay.
Some lecturers at a few colleges walked out over claims a pay package agreed last year has not been honoured locally.
Colleges complain they get less funding per student than schools.