The Federal Court of Australia has rejected an employer group's attempt to break up the merged Construction, Forestry, Maritime, Mining and Energy Union.

The militant CFMEU created a new $150 million "super union" when it merged with the Maritime Union of Australia and the Textile Clothing and Footwear Union in March, sparking employer fears that it would be further emboldened to break industrial laws.

CFMEU national secretary Michael O'Connor and MUA national secretary Paddy Crumlin have defeated a legal challenge to their union merger. Credit:Nick Moir

CFMEU national secretary Michael O’Connor said the Federal Court decision to uphold the merger not only vindicated the union’s position, but showed employer groups were "wasting their members' money waging an ideological battle" against it.

"Our argument throughout has been that the democratic will of union members should decide how our union is structured, bargains, and represents their interests, not employer groups using legal technicalities to serve their vested interests," Mr O’Connor said on Friday.