Horta-Osorio went on a leave of absence on 2 November after taking medical advice but said he planned to return by the year end.
Tim Tookey, group finance director at LBG is currently interim chief executive but he is due to leave the group in February 2012 to take up the CFO role at Friends Life.
Roberts joined the LBG board as a non-executive director in March 2010 and is chairman of the risk committee. He has also previously held board positions at Barclays.
Nathan Bostock, RBS’ head of restructuring and risk, has also revealed he plans to stay at RBS rather than joining LBG as previously announced in July this year.
LBG chairman, Sir Win Bischoff, said: “The board is pleased that António is making good progress and we are looking forward to his return at the end of the year. However, we recognise it is important that we formalise the contingency arrangements if António’s return is delayed.”
