More than one-quarter of licensed premises in London were found to have sold alcohol to children.
Tests by London Trading Standards Authorities showed that 26.8% of licensed premises were guilty of the offence, compared with 22% in 2005/6.
The tests involve local authorities arranging for children to attempt to buy alcohol in pubs and off-licences.
Despite the increase, only 10% of the on-trade outlets responsible were prosecuted. Some 58% were fined, and a further 10% were cautioned.
This week, PM Gordon Brown met with industry chiefs to discuss how best to tackle the problems of underage sales and binge drinking.