More than a million students could face disruption to lectures and classes today as university workers begin a wave of walkouts in an ongoing bitter dispute over pensions.
Members of the University and College Union (UCU) at 64 UK institutions are staging the first in a series of strikes after voting in favour of industrial action.
Tens of thousands of workers are expected to take part in the walkouts, which have been described as "disappointing" by university bosses.
The dispute centres on proposals put forward by Universities UK for changes to the Universities Superannuation Scheme (USS).
Employers argue that the pension scheme is billions of pounds in deficit, while the union says the proposals would leave a typical lecturer almost £10,000 a year worse off in retirement.
Strikes will take place today and tomorrow, with more in the coming weeks if there is no resolution, building up to a five-day walkout in the week beginning March 12.
UCU said it expected that tens of thousands of its members will take part in the action, with over a million students affected and 575,000 teaching hours lost.
There have been reports that thousands of students have signed petitions demanding compensation from universities for classes lost due to the strikes.
UCU general secretary Sally Hunt, said: "The scale of these unprecedented strikes reflects just how destructive the proposals would be for staff pensions and their anger at university leaders to come back to the table to negotiate.
"Despite the fact that over a million students are going to be affected, university employers have been unwilling to reconsider their position and look at reasonable alternatives which would give staff security in retirement.
"Nobody is taking this action lightly, but the ball remains firmly in the employers' court. If further disruption is to be avoided, university leaders must put further pressure on their representatives to get back to the table for meaningful discussions with UCU."
UUK said that the pension scheme has a deficit of more than £6 billion that cannot be ignored, and there is a legal duty to put a credible plan in place by the summer to reduce the deficit.
A spokesman said UUK had met UCU over 35 times in the last year to discuss reforms.
He said: "UUK remains at the negotiating table, but so far UCU has refused to engage on how best to address the funding challenges facing USS. It is important now that UCU engages on how best to ensure the long-term sustainability of the scheme."
He added that there are scheduled discussions with UCU on key issues with the USS.
The spokesman continued: "The changes proposed will make USS secure and sustainable, safeguarding the future of universities.
"University staff will still have a valuable pension scheme, with employer contributions of 18% of salary, double the private sector average. This makes strike action very disappointing."
Around 16% of academic staff that are UCU members in the 64 institutions affected voted in favour of strike action, according to UUK.
On the eve of the action, Universities Minister Sam Gyimah warned UUK and UCU that they must resolve the "damaging and avoidable impasse".
In a tweet referencing an editorial in the Times newspaper, the minister said: "Times Editorial on Uni strikes is spot on. For the sake of students and the reputation of our Universities, @ucu and @UniversitiesUK need to find a way through this damaging and avoidable impasse ASAP."
UCU has warned that if the dispute is not resolved, then action could continue, including into the summer exams period.
Union officials are meeting on March 2 to discuss the response from universities to the industrial action, and Ms Hunt has warned that if the dispute is not settled then "nothing is off the table".
The union has a mandate to take industrial action up until July 19, which covers the period when exams are being taken by students and marked.
Hundreds of thousands of university workers are understood to be enrolled in the USS pensions scheme, mainly those working at older institutions established before 1992.