Media unions have vowed to fight a labour-law clause which allows bosses to set up their own in-house unions.
The National Union of Journalists and print union Unite are determined to take the issue to the EU - but fear even a positive result wouldn't be fully enforced.
Unite assistant general secretary Tony Burke said existing laws allow firms such as Rupert Murdoch's News International to set up and recognise their own toothless unions, keeping out independent ones.
"We have been in discussion with Professor Keith Ewing, an expert on employment law, and he is of the view this is contrary to International Labour Organisation conventions on the right to organise free trade unions," he told the Morning Star.
"The problem is that things grind very slowly and in the end we might get a good result but whether it would be enforceable I don't know."
He was speaking at the TUC's Taking on the Media Barons conference in London on Saturday
He said: "The News International scandals and all the other issues have made people change their minds about media ownership in this country.
"We have got to put pressure on the Labour party - Ed Milband got off to a good start by distancing the party from Murdoch - to come out very strongly that it is opposed to concentration of media ownership.
"We cannot allow one media conglomerate to own so much of the media in any one country."