Hundreds of lecturers in 64 of Britain’s top universities, including Oxford and Cambridge, began a two-day strike on Thursday to ward off changes to their pension scheme.

The lecturers belonging to the University and College Union (UCU) voted to strike to protest planned changes to pension benefits, claiming that some staff could lose up to 10,000 pounds (14,000 dollars) annually after they retire.

The union said it expected the strike, which is scheduled to be followed by three longer ones during the next month, to affect more than 1 million students.

The UCU accused the universities of refusing to discuss the lecturers’ concerns and seeking to “impose their reforms,” adding that it hoped that “no universities would want a prolonged dispute that drags out towards exam season.”

“The key is how universities react to the action this week,” said UCU general secretary Sally Hunt.

“We will be meeting on 2 March to consider what wave two of the action may need to involve, and nothing is off the table,” Hunt said.

Universities UK, which represents the universities, said they were “making every effort to minimise the disruption to students” on Thursday and Friday.

The union said there were “a number of misleading claims” about the planned changes to the pension scheme, which had been “affected by difficult economic conditions.”

“The cost of future pensions has risen by one-third in the last three years, and the scheme has a deficit of 6.1 billion [pounds], which, by law, must be reduced,” Universities UK said.