Punters to splash out £200m this weekend

Britain is set for its biggest ever weekend of gambling with bookies expecting more than £200million to be staked on a triple bill of sport.

The bets will be placed on Saturday's Grand National, football's two FA Cup semi-finals and the crucial Third cricket Test between England and the West Indies.

The growth in internet betting means about 30 per cent of the wagers, worth some £60million, are expected to be placed online.

However, the run-up to the big weekend is overshadowed by fears that some gambling websites are being targeted by cyber-extortionists threatening to paralyse their operations with millions of bogus emails in the key betting hours. Pete Simpson, director of computer security company Clearswift, said: "The sites at the centre of the extortion attempts have been unusually quiet. It could be that they have paid up but just don't want anyone to know.

"Alternatively they may be working with police who are tracking any money they pay."

Several companies are known to have been the target of blackmail attempts by hackers based in Eastern Europe and Russia. They include Stanley Racing, Paddy Power and bet365, although all refused to hand over any money. Even larger firms such as William Hill have been the targets of attempted attacks.

The hackers are believed to have been encouraged by their success in persuading some Caribbean-based sites to pay about £16,000 each.