A third of large companies in the UK have experienced attempted hacks on their web sites, but almost three quarters are confident of their security defences, according to research.

The companies affected are being hit by one attempted hack every week, on average, according to the biennial Information Security Breaches survey from the Department of Trade and Industry and PricewaterhouseCoopers (PwC).

Four per cent of attempts were successful, and victims rate them as their most serious breach, well ahead of viruses.

Around half of businesses have their web sites hosted externally. Firewalls are the main security defence for most companies, with half citing them as their only defence, according to the research.

'The survey findings point to a real concern that businesses without the right monitoring information and intrusion prevention processes in place may have a false level of comfort,' said Andrew Beard, the PwC advisory services director involved with the report.

'Scanning and hacking activity may not be detected until it is too late to react,' he said.

The full results of the survey will be announced at the InfoSecurity conference in London in April.