The National Hi-Tech Crime Unit (NHTCU) is investigating a number of blackmail attempts targeting online bookies.

The threats, believed to be originate from eastern European gangs, have demanded bookmakers pay up to £30,000 each or face distributed denial of service attacks (DDoS).

The DDoS attacks force websites to close for hours, as targeted sites are flooded with so much information that customers cannot access them.

Last month, Paddy Power, an Irish internet bookmaker, was attacked shortly before the Super Bowl. The company’s website was forced out of action for hours and resulted in loss of business on a normally busy betting night.

The news comes at a time when the European online betting sector – believed to be worth around £7bn – is lining up for a busy year with a number of major sports events, including the Grand National and Euro 2004, just around the corner.

The NHTCU said it believed that “specific sporting events are being targeted.”

Mick Deats, head of operations, said: “It’s not clear how much research the hackers are doing, but in Paddy Power’s case, there was an awareness it was the Super Bowl and a good time to attack. There’s a string of large betting events coming up this year."