Ictu General Secretary David Begg has welcomed the decision of the Association of Secondary Teachers of Ireland (ASTI) to apply to rejoin Congress.
Asti left Congress six years during a protracted dispute over teachers' pay but today it announced a ballot of members overwhelmingly supported re-affiliation.
Balloting of the union's 17,000 members began at the start of the month following a lengthy consultation period and today it was announced that 79 per cent supported re-affiliation with 21 per cent against.
Asti general secretary John White said rejoining Ictu would help the union lobby Government to address under investment in second level education.
Mr White said: "Teachers by the nature of their work are deeply involved with the broader community and this vote of ASTI members to re-affiliate to Congress means that we will be able to work together with the broader trade union movement for the good of teachers and the education service.
Later, Mr Begg said Congress would be strengthened by Asti's presence and promised the Ictu executive would deal with the application speedily.
"The effectiveness of the trade union movement in bringing about positive social and economic reform is directly related to its internal cohesion and sense of common purpose ... the return of the Asti will further strengthen its position", Mr Begg said.
The union was forced to leave Congress over the 30 per cent pay rise it was seeking for members during a bitter dispute with the Department of Education.
The Asti position angered the Government and other unions because the pay claim was far beyond provisions of the social partnership.