London firm Reid Minty is suing construction union UCATT (Union of Construction, Allied Trades and technicians) for alleged unpaid fees totalling more than £80,000.
It is understood that the firm's action against the 157,201-member union dates from 1992 and includes bills for work on successful libel cases brought against rival union officials and a building sector magazine.
Reid Minty litigation partner Simon Moore is understood to be acting for the firm, but neither he nor any other partners were available for comment.
The writ, issued at the High Court in London, has taken UCATT officials by surprise, according to comments made in the magazine Construction News.
Reid Minty is thought to be one of three law firms which regularly act for UCATT, particularly on personal injury cases.
The two other firms are Christian Fisher & Co and OH Parsons & Partners.
Parsons is defending UCATT against the Reid Minty writ.
Parsons partner Barry Sullivan was not available for comment.
Based in London, UCATT is a long-established trades union operating on behalf of the building and construction sector.
A spokesman from Reid Minty was quoted in Construction News as saying: “This is a very strange situation after all the years we have worked with them.
“But we have unpaid bills and it is not clear what will happen with the cases we are currently working on for UCATT.”