The first posters warning travellers not to drink alcohol on London's public transport network have already been put up.

Tickets will be rewritten from 1 June to bar passengers from carrying open bottles or cans of alcohol on buses, the Tube, the Docklands Light Railway and trams as part of Mayor Boris Johnson's drive to deal with petty crime. Anyone caught drinking alcohol can be asked to stop, or be removed if they refuse.

A by-law that could impose fines for offenders is expected to be passed next year, making drinking on the transport network a criminal offence. Transport police, unions and alcohol charities welcomed the new powers, |arguing that most people would comply with the ban.

The ban on drinking was a manifesto pledge by Mr Johnson, who has made tackling crime a key policy of his first days in power at City Hall - but the plans were criticised as unworkable by some. Tom Brake, the Liberal Democrat spokesman for London, said: "There is no point introducing a law which everyone disregards and no one seeks to enforce. Boris Johnson must explain why this is something more than just a spin doctor's wheeze."

Mr Johnson said: "I'm determined to improve the safety and security of public transport in London and create a better environment for the millions of Londoners who rely on it. I firmly believe that, if we drive out so-called minor crime, then we will be able to get a firm grip on more serious crime."

Don Shenker, chief executive of the charity Alcohol Concern, said: "Public drinking and the behaviour associated with it can, and does, deeply affect people's ability to enjoy public spaces. Taking a firm approach to public drinking in this way sends a strong message that public drunkenness is socially unacceptable, and will support both the public and transport staff."