By Harry Wilson, Banking correspondent

An unknown group launched a so-called “denial of service” attack on Britain’s largest bank on Thursday evening, crashing web-based services for about seven hours.

At its peak, HSBC was deluged with internet traffic 500 times its normal level, five times higher than the amount of traffic that hit the bank in a similar attack earlier this year.

A spokesman for the bank said full internet services were restored by 3am yesterday and claimed that no customer data had been compromised as a result of the attack.

“We are co-operating with the relevant authorities and will co-operate with other organisations that have been similarly affected by such criminal acts,” HSBC said. “We apologise for any inconvenience caused to our customers throughout the world.”

US financial group Capital 1 is also understood to have been hit by a similar denial of service attack at the same time as HSBC.

Several parties have claimed responsibility for the attacks, which have been linked to Islamic groups protesting against controversial film The Innocence of Muslims, which they want removed from the internet.

Businesses increasingly see cyber attacks as one of their biggest threats given the migration of consumers online.

The Government issued advice to British business leaders last month on how to protect themselves from such threats.