MADRID, Sept 2 (Reuters) - Spanish telecom operator Telefonica (TEF.MC) has hired investment banks Barclays and LionTree to seek a partner to finance its fibre network in Britain, Spanish online newspaper El Confidencial reported on Thursday citing sources close to the transaction. British cable and mobile provider Virgin Media O2, jointly owned by Telefonica and Liberty Global"s , is planning to upgrade it entire network to full fibre by 2028 and raise its number of subscribers to 21 million, El Confidencial said. The operation would require between 4 billion euros and 5 billion euros, El Confidencial said. A spokesperson for Telefonica declined to comment. A Barclays spokesperson was not immediately able to comment. LionTree did not immediately respond to a message seeking comment.