Thousands of Virgin e-mail users are being issued with new passwords after the company found a hacker had been attempting to tap into its mailing system.

More than 170,000 of Virgin Net's 800,000 UK customers had their service temporarily withdrawn at the weekend.

A notice was posted on Virgin's official website warning users of the potential breach, and giving step-by-step instructions as to how they could change their passwords.

Individual letters were also being sent out to inform anybody who had not logged on since the problem was detected.

A spokesman said on Monday: "No actual security breach has happened, but we discovered someone was attempting to hack in.

"Because we were able to work out how they were trying to do this, we were able to isolate a maximum of 25% of our customers who might have potentially been affected.

"Their e-mail facilities have been temporarily switched off and we are in the process of creating new passwords for them.

"It is important to emphasise that, in the event, no-one's security has actually been breached."

The spokesman added that all those concerned were a certain "type" of customer, but he declined to identify which one.

Microsoft scare

The Virgin security scare comes just four months after Microsoft was forced to temporarily shut down e-mail links for 40m customers worldwide, following a breach of the company's Hotmail security system.

On that occasion, a group of seven programmers calling themselves Hackers Unite later e-mailed online news service Wired to claim responsibility for the breach, which they said was intended to demonstrate the inadequacy of Hotmail's defences.

Virgin Net user John Holland learnt of the potential breach when he tried to retrieve his e-mail messages on Sunday, and found his password was being rejected.

He said: "For me it wasn't such a big problem, but for some customers who are trying to run businesses using e-mail they could have missed out on dozens of messages by the time they receive their letter telling them about the situation.

"A potential breach doesn't bother me particularly because I don't have that much confidential information coming over, but clearly the fact there has been a potential breach is a cause for concern to some people."