By Paul Reynolds
World affairs correspondent, BBC News website
Robert Mugabe has run Zimbabwe for 27 years
The British and some other Western governments believe that the most likely way for President Robert Mugabe to leave office in Zimbabwe is by a "palace coup" led by factions in his own party.
A military-type coup is thought to be unlikely. There would be an accumulation of overwhelming pressure instead.
However, it is also accepted that he might face down his critics and contest and win another six-year term as president next year, despite being already 83.
Opposition too weak
Foreign diplomats do not appear to think that the opposition leader Morgan Tsvangirai is strong enough at the moment to effect a change. "The opposition was swept off the streets," said one.
They are therefore looking to people inside the ruling Zanu-PF party.
The names of a former general, Solomon Mujuru (whose wife Joyce is a vice-president), and Emmerson Mnangagwa, a former head of security, are being mentioned as possible future leaders, with a former finance minister, Simba Makoni, as a prime minister.
And signals are going out that there would have to be a change of direction not just of personality for Zimbabwe to re-enter the international fold.
"2007 is a pivotal year", a senior British official told reporters in London.
The problem with that statement is that there have been pivotal years before and nothing has changed.
This time, in the British view, it is different.
There is the 2008 election coming up and that requires decisions.
There is economic catastrophe, in which inflation could reach 5,000% later this year.
Opposition leader Morgan Tsvangirai is not seen as strong enough to withstand Mugabe
There is internal dissent within Zanu-PF, which in December refused Mr Mugabe's request to stay on until 2010.
There is civil unrest and there is Zimbabwe's international isolation, including growing isolation from its neighbours.
The British government believes there are several scenarios for a Mugabe exit - he could negotiate his departure, he could be pushed out or there could be a civil explosion.
The most likely scenarios are seen to be the first two, his departure engineered in some way by his own party.
And Western governments are now drawing up what they called "principles of re-engagement".
These are the basic conditions under which they would help a Mugabe successor.
A new leader would have to stabilise the economy, by ceasing to print money for a start, return to a rule of law, end state violence and eventually hold an election.
The concept of a peacekeeping force to help in that process has not been ruled out.
Britain is quietly helping human rights lawyers, though it denies funding the opposition.
It is also gathering information about violence by the government, has given aid totalling £150m over the last few years and is encouraging Zimbabwe's international isolation.
However, it does not want to give Mr Mugabe a stick with which to beat it, so is preventing its ambassador Andrew Pocock, who is in London at the moment, from speaking out publicly, unlike his American counterpart Christopher Dell.
One obstacle to bringing international pressure on President Mugabe is that he is regarded as the liberator of southern Africa.
South Africa has tried quiet diplomacy but is unwilling to engage in public condemnation.
It could cut off electricity to Zimbabwe but is reluctant to do so.
The British hope may sound far-fetched but it is that South Africa will want change in time for the football World Cup it is hosting in 2010.
But nobody is counting on such change.