Richard Marwood, senior fund manager at Royal London Asset Management, said the news was an unwelcome addition to existing concerns about BT's pension liabilities and relationship with the industry regulator, OFCOM.

"Today's announcement from BT surprises us on a number of counts. Firstly, BT is a strong company with a relatively predictable business and so not generally prone to these kinds of warnings," Marwood said in a statement.

"Secondly, given the modest scale of the Italian business, the magnitude of the hit it has caused is concerning. Finally, the warning that spending by government and corporate customers has shown some signs of softening is the sting in the tail of the announcement."

Royal London Asset Management said it currently holds 89,305,993 shares in BT worth over 340 million pounds, giving it a 0.9 percent stake in the group. This would make it one of the 10 biggest investors in BT, data from Reuters showed.

(Reporting by Simon Jessop, Editing by Maiya Keidan)