Many web site home pages were replaced with the message, "looking for your homepage? It has been taken off the server. Nothing personal but this has been done to expose Cable & Wireless's poor security."
The hacker also announced that he had the personal details of 150,000 Cable & Wireless customers.
Cable & Wireless, which is currently investigating the situation, has however issued a statement assuring customers that their personal information is held on an entirely seperate computer system.