The Unite trade union has been told by the Irish Congress of Trade Unions it should terminate the membership of its crane operators and encourage them to rejoin SIPTU.
It follows a complaint made by SIPTU that Unite had signed up workers who were already SIPTU members.
The findings are contained in a report drawn up by ICTU's Disputes Committee.
The committee found that Unite had contravened Article 46 of the Congress constitution and said Unite should not seek to organise or represent crane operators, banksmen or related grades.
It said Unite should also amend its membership application form to comply with Congress rules.
The committee said it accepts that SIPTU is the representative union for the general operative grades in the construction sector nationally except for certain specific geographical areas.
Tom Fitzgerald, regional officer of Unite, said the likelihood is that the union will appeal the findings.
He said in terms of the list of concerns construction workers have, this will be categorised by them as a sideshow.
He said his union had around 173 crane operators on its books, representing over 90% of the total number of such workers.
He said it should be their decision which union they want to be a member of and said Congress should support that.
SIPTU's divisional organiser, Greg Ennis, has welcomed the findings of the disputes committee and said SIPTU will continue to strive to improve the terms and conditions of all construction workers including crane operators.