"I WILL NOT GROW OLD"
Mugabe has previously said he did not want to name an heir over fears he or she would become a target of other officials nursing ambitions to succeed him as ZANU-PF leader.
The president gave no timetable for his possible retirement and added: "But as long as the British still want to come here, I will not grow old; until we know we no longer have sellouts among us."
Mugabe this week threatened that he and his independence war veterans will take up arms again to stop the MDC taking power.
The MDC and rights groups say ZANU-PF have launched a brutal campaign of violence which has killed at least 66 MDC activists, wounded hundreds others and displaced tens of thousands since the March 29 election.
Britain's Miliband said South Africa had a responsibility to do more to bring pressure on its neighbour, and condemned the violence that has marred the run-up to the election.
"The first thing is to be clear about the sadism, and I use that word advisedly, that's going on ... in Zimbabwe," he told BBC television.
"People being killed, people being tortured, people being beaten. Election observers being stripped out, election officials being stripped out."
The African Union expressed concern over reported violence and said it planned to send "a sizable" team of observers to monitor the run-off poll.
Tsvangirai says he is confident of victory despite the intimidation campaign in which he has been detained several times this month.
