Capital Sports' website has been the worst affected and has been unavailable since mid day yesterday.
A spokesman for Blue Square said the company came under attack after the company ignored a demand for $30,000. The new attacks raise speculation that extortionists will continue to target gambling sites in the run-up to Euro 2004 and Wimbledon.
Netcraft is monitoring the performance of twenty leading UK Internet Gambling Sites, with dynamically updating graphs available here.