Marks & Spencer today confirmed that it was probing an apparent attempt to access the phone records of new boss Stuart Rose.
It is thought a third party was able to register a pass code to view his call records held by mobile phone firm mm02.
It is unclear whether this unknown person was successful in obtaining the records, which could be crucial to an investigation by London's City watchdog into dealings in M&S shares.
Fears that sensitive information was leaking into the public domain led M&S to hike security around Mr Rose and his advisers just 24 hours ago.
An M&S spokesman said: “We can confirm that it appears someone has accessed Stuart Rose’s telephone records and we are looking into that further.”
M&S has twice rejected takeover approaches from retail tycoon Philip Green, who is willing to offer more than £8bn (€11.9bn) for control of the retailer.
There is no suggestion that Mr Green, or anyone connected with him or his bid, is connected to the apparent breach of security.