Children as young as 11 are able to gamble online, according to a new report.
Many online websites fail to carry out proper age checks
They are able to do so because of failures by a number of websites to carry out proper age checks.
The vast majority of sites tested by children's charity NCH allowed under 18-year-olds to register their details, the report found.
The charity is now urging gambling websites to apply stringent age verification software.
A 16-year-old girl was asked by the charity to attempt to register with around 37 UK gambling websites.
She claimed she was 21 and gave her Solo debit card details.
As a result, the teenager was able to register her details with 30 UK websites and she would have been able to place bets.
Gambling Minister Andrew McIntosh said: "These are very worrying findings.
"Having already warned the industry that Solo cards should be treated with caution, it is disappointing to find so many haven't taken this on board".
Natwest offers Solo debit cards to customers over 11 years of age and HSBC does the same for any account holder aged 16.
John Carr, NCH's internet adviser, said: "It is shocking that children as young as 11 are able to register with online gambling sites. There are no excuses for this.
"The technology for these companies to clean up their act already exists but it is used by a very small number of the operators we surveyed".
UK websites will be required to make proper age verification checks under the Gambling Bill, which is set to be introduced in the autumn.
The report was compiled with CitizenCard, an organisation that supplies ID
cards and which is also behind age verification software being tested by
gambling websites - including some of those which failed the test.
One of the 30 websites which the report said allowed the 16-year-old girl to register is run by bookmaker William Hill.
Spokesman David Hood said: "Issues such as under-age gambling are hard to address but they are being addressed.
"We are testing several types of age verification processes.
"However, we still believe the burden of responsibility lies with banks
because issuing these cards to minors gives them access to all types of adult
services, including porn sites."
The National Lottery was one of the sites which did block children when tested.
A spokeswoman for operator Camelot told BBC News Online up to 10% of internet gambling applicants are excluded because their age and UK address cannot be confirmed.
Banks 'could help'
"We've invested a lot to develop the appropriate software to prevent underage and excessive play," she said.
"We wouldn't launch online until we were sure we had the appropriate controls in place."
Ladbrokes also blocked under-age use successfully.
E-gaming director David Briggs said the company felt there was a greater risk customers holding switch, solo and electron cards were under-age so they are checked against a data-base of over-18s in the UK.
"It's a pretty inexact science, it's difficult to do with 100 per cent certainty," he said, but added thousands of accounts had been investigated and suspended.
He called on banks to help companies identify under-age card holders by including a code in online transactions which specified whether they were under or over-18.
The gambling findings come seven days after a similar survey found parents are still largely unaware of the risks their children take on the net.
A London School of Economics study suggested 57% of children had seen net porn while 16% of parents thought their children had.