Virgin.net has pleaded with its customers to "bear with us" as it struggles to overcome a potential security breach, which has left more than 150,000 customers without access to email accounts.

Frustrated Virgin.net customers have been forced to change their email passwords after last weekend's incident, which Virgin says is being investigated by police.

One businessman from Yorkshire said he planned to cancel his subscription with Virgin after four days of trying and failing to contact the company's customer support line to reactivate his email account.

"We've been on the phone to Virgin constantly and cannot even get through at five in the morning," said Robert Minton-Taylor, who runs the Minton-Taylor public relations consultancy in Skipton, Yorkshire.

"I appreciate all companies have the occasional problem but our firm relies on Virgin.net for 80% of our business communication needs.

"We will be switching to another provider."

A spokesman for Virgin.net said that capacity at its customer support call centres had been increased and 166,000 of its subscribers had been sent letters in the post explaining how they could get their email accounts up in working order again.

"We apologise to our customers and ask them to be patient," the spokesman said.

"We are throwing every resource we can at solving the problem and working through the night to get it fixed. It is a mammoth effort.

"Customer security is of paramount importance to us and we want to ensure everything is done properly."

Virgin.net, which has 620,000 users, said last night that 40% of affected users had been given working passwords and the rest would be able to send emails by the start of next week.