More disaffected students have joined a campaign urging lecturers to stop industrial action which could see exams severely disrupted this summer.
Students fear the action will stop them graduating
Members of Natfhe and the Association of University Teachers are refusing to mark work and exams in response to a "derisory" pay offer from employers.
Last week 21 university student unions criticised the National Union of Students for supporting the boycott.
Now a further 10 student unions have
joined a lobby against the action.
The NUS maintains that the majority of its campus members support the action and says more than 30 student unions have signed a counter-protest, backing the boycott.
"Just yesterday University College London Students' Union, who previously signed up to the letter condemning AUT, passed policy to support the union and their action," said NUS president Kat Fletcher.
Lecturers have held a one-day strike and are boycotting students' exams
Ms Fletcher said, in supporting the AUT action, the NUS was "representing the interests of all of our members".
"Well-paid staff are motivated staff who will go the extra mile in delivering a high standard of service to students," she said.
And the NUS would continue to urge the AUT to set exam papers, having "always opposed AUT's tactic to not set exams", she added.
But a spokesman for the growing number of disenchanted student unions said: "It is a regrettable fact that the NUS chose to adopt a pro-AUT position on the issue of the boycott.
"This was a severe error of judgement by the NUS."
Students meet union leader
On Thursday, three representatives from the "rebel" student unions met with the general secretary of the AUT, Sally Hunt, to urge her to call a halt to the action by lecturers.
Gaston Dolle, Andy Wilson and Alain Desmier, from Bristol, Southampton and Exeter students' unions respectively, said they were not against a pay rise for lecturers, but opposed the way in which the boycott was affecting students, especially finalists.
But the meeting failed to produce a change of heart on either side.
"It was an interesting meeting, but certainly not a productive one," said Mr Dolle, president of Bristol students' union, after attending the meeting.
A spokesman for the AUT said the student representatives had shown a lack of understanding of the way the AUT operated and insisted the union was "willing to have a discussion".
The spokesman said any actual or potential harm to students as a result of the boycott was "with regret".
"We are still looking for the employers to acknowledge and engage with our claims," he added.
The AUT and Natfhe have rejected as "derisory" a pay offer by the University and Colleges Employers Association (UCEA) of 3% from August this year and an extra 3% from August 2007.
The latest talks in the pay dispute earlier this week failed to lead to formal negotiations.