One in four retailers tested in sting operations have been caught selling tobacco to under-age youngsters.
When Trading Standards officers carried out a series of random checks across Portsmouth after the raising of the minimum legal age for buying tobacco from 16 to 18, 26 per cent of the shops tested failed.
None of the retailers has been fined for the failure to comply, but they are being warned that next time they will end up in court.
'I'm extremely disappointed, especially considering there's been so much press about the change in law,' said Robert Briggs, manager of Trading Standards at Portsmouth City Council.
'There's absolutely no excuse for not knowing – retailers could face the courts.'
Fifty shops were randomly tested across Portsmouth on three separate days.
On day one, two 15-year-old girls were sent into 18 premises without ID. Two shops failed the test and served the girls cigarettes.
On day two, a girl aged 15 and a boy aged 16 entered 18 shops. The pair both had ID, yet even when their ID was checked, four shops still failed the test by continuing with the sale.
And finally on day three, two boys aged 15 visited 14 shops, with ID, and half of those shops served them.
Mr Briggs said: 'This time they got a warning, next time we'll take action.
'Traders need to be very careful. My message to them is check for ID if you think someone looks under 21.'
The law on tobacco sales changed on October 1, making it illegal to sell tobacco to anyone under 18.
Traders across the city were informed by Trading Standards, monthly traders magazines and even by cigarette manufacturers who visited individual stores.
Graham Hales, member of the National Federation of Retail Newsagents and owner of Quix newsagents in Portsmouth Road, Cosham, Portsmouth, said: 'It was definitely well-publicised, so I agree there's no excuse for not knowing.
'If anything it was the kids that didn't seem to know, I don't think it was very well-publicised to them, so it was hard in the first few days and we had a few of them mouthing off.
'But word got around quite quickly and once they knew, they were OK.'