Easing the restrictions on glass sizes for draught beer and cider is designed to appeal to people who can't face a pint, but think a half is too small.
Currently, pubs can only serve pints, half pints and a third of a pint, a measure rarely seen in pubs but popular at beer festivals where drinkers try lots of different ales.
A British Beer and Pub Association spokesman said the proposal was a good idea that would appeal to women who may not want a full pint and also to people drinking strong beers or specialty ales.
"Why not give customers the choice so they can buy whatever size of drink they like?" the spokesman said. "At the Great British Beer Festival in July, the third of a pint glass was in very, very common use.
"It encourages people to experiment with different beers. It's something we'd like to see more of."
The National Weight and Measures Laboratory, the government body which enforces the rules, is consulting the public and industry on the changes.
It said the proposal would give pubs more flexibility, but warned that people ordering two-thirds of a pint may find it harder to keep track of how much they have had to drink.
Last November, the European Parliament voted to scrap a law that would have phased out imperial measurements such as the pint in Britain and replaced them with the metric system.