LONDON (AP) _ Thousands of Virgin Net e-mail users were given new passwords after the company uncovered a potential security breach.

More than 170,000 Virgin Net customers had their service temporarily withdrawn after technicians discovered a hacker attempting to tap into the British-based system, the company said.

A notice was posted on Virgin's Web site this weekend warning users of the potential breach, and individual letters were mailed to all those affected, the company said.

Only about 25 percent of Virgin's customer base was potentially involved, because the hacker's efforts were confined to a smaller area, the company said Monday.

Virgin Net, which provides e-mail services for 800,000 British customers, said the accounts potentially affected all belonged to a certain ``type,'' but would not elaborate.

The scare comes four months after Microsoft was forced to temporarily shut down e-mail links for 40 million customers worldwide following the detection of a security flaw in its Hotmail security system that rendered users vulnerable.