Owen Warnock, partner in international law firm Eversheds, said it would not be easy to introduce the labels, which give guidance on the alcohol content of individual packs rather than an abv level.
Warnock's comments came after the Royal College of Physicians called for compulsory unit labelling when National Statistics figures revealed that more than a third of British adults were exceeding the Government's safe daily drinking guidelines at least once a week.
Doing so could create a barrier to free trade because suppliers outside the UK would have to produce UK-specific labels, warned Warnock.
The alternative would be to claim special exemption from the legislative rules in the interests of public health, he added.
"It might face a legal challenge from the EU to test whether the health grounds are sufficiently compelling to enable a special exemption to be used," he said.
The option to include the measure in a draft EU-wide Food Information Regulation is currently
Professor Ian Gilmore, president of the RCP, said: "Knowledge of those limits is poor, despite 10 years of concerted work to raise awareness. It is vital that the Government takes the next step of introducing mandatory labelling on drinks."
Many drinks brands and own-labels already include unit labelling on packaging on a voluntary basis.