The UK's hi-tech crime squad has confirmed it is looking into blackmail attempts, thought to be by Eastern European hackers, to betting websites.
The gangs are trying to disrupt web betting
The gangs have threatened to flood bookmakers' sites with data, disabling them before big events like Euro 2004.
A National Hi-Tech Crime Unit spokesperson said it had started investigating the threats last autumn.
An Irish bookmaker said it had been attacked electronically on the night of the Superbowl last month.
"We are aware of it and we are investigating," said the NHTCU spokesperson.
"We know that specific sporting events are being targeted. This could well include the Grand National and Euro 2004," she added.
The threats, made by e-mail, have demanded that bookmakers pay up £20,000 to £30,000 or face distributed denial of service attacks.
These attacks work by flooding targeted websites with so much information so that no one can access the sites.
If this happened, it is estimated it would cost bookmakers hundreds or thousands of pounds in lost bets.
Last month, Irish bookmaker Paddy Power, was targeted on the night of the Superbowl, paralysing its site for several hours.
A spokesman said it had received an e-mail threatening an attack, but had not been asked for a specific ransom.
"As soon as we discovered what it was, we got straight on to the police," he said.
"We were never going to pay. It is a pretty cheap and nasty way of trying to extort money."
Similar denial of service attacks were attempted on software sellers SCO and Microsoft's websites earlier this year, although no blackmail attempts were linked to the attacks.