Telefonica is considering "all strategic choices" for its British operations, including O2, as it seeks to reduce its debt. John D Mchugh/AFP
BT added in a statement that it had been approached by shareholders from two operators — O2 and an unnamed group — with a view to buying their UK mobile business.
The other operator was understood to be EE, according to sources close to the matter. EE is a joint venture between Orange of France and Deutsche Telekom of Germany.
The four biggest mobile phone groups in Britain are O2, EE, Vodafone and Three, which is owned by Hong Kong's Hutchison Whampoa.
Back in 2002, BT demerged its O2 division, formerly known as BT Cellnet, before it was purchased by Telefonica in 2005 for £17.7 billion ($27.8 billion).
Last week however, Telefonica indicated that it would consider all strategic choices for its British operations, including a possible sale.