LONDON (Reuters) -Thousands of British university staff have voted to strike in a dispute over pay, the trade union Unison said on Thursday.

The latest announcement adds to growing industrial unrest across Britain, with rail workers, health staff and legal professionals striking or planning strikes to demand better pay and conditions.

University staff including cleaners, administrators, library, catering and security workers will walk out after rejecting a 3% pay offer from the University and Colleges Employers Association, the union said.

"This ludicrously low increase does nothing to ease the financial pressures for thousands of struggling staff," Unison's head of education Mike Short said.

The union said staff in more than 20 universities across the country, including employees from four Scottish universities, one in Northern Ireland and 17 in England, will take part in the strike action later this year.

Institutions based in London, Manchester, Liverpool, Leeds, Bath, Belfast, Edinburgh and Glasgow will all be affected, Unison said.

(Reporting by Farouq Suleiman; editing by William James)