Members of the Association of Teachers and Lecturers have voted overwhelmingly to affiliate to the Trades Union Congress.
The move was agreed by a 72 to 28 per cent majority. Turn-out was low with less than a quarter of the union's 112,000 members voting.
Peter Smith, ATL's general secretary, said TUC affiliation would give the union a stronger national voice and access to policy-making decisions in Europe.
"ATL will fully retain its independence and reputation for its strictly non-party political stance. TUC affiliation will strengthen ATL's voice, not muffle it," he said.
Affiliation will cost an extra Pounds 1.70 per member, which will be covered by efficiency savings.