Zimbabwe will be offered a seat back in the Commonwealth within two years as long as it agrees to political and economic reform.
President Robert Mugabe pulled the nation out of the Commonwealth, which he denounced as an "evil organisation", in 2003.
But Prime Minister Morgan Tsvangirai supports a return, and Britain believes the troubled nation can be brought back into the mainstream world community.
The meeting of the Commonwealth Heads of Government, which opens in Trinidad on Friday, will send a message to Zimbabwe's leaders that a place at the table could be possible at the next gathering in late 2011. However, Zimbabwe, which is politically deadlocked, needs to clear a succession of hurdles before it can return.
The Southern African Development Community (SADC) has set a 6 December deadline for Mr Mugabe's Zanu PF and Mr Tsvangirai's MDC to break their impasse on sharing power, and the Commonwealth heads will insist that Zimbabwe's leaders stick to that date.
They are also likely to call for reforms to the political, economic and legal systems to enable the nation's return to the Commonwealth.
The subject of Robert Mugabe has overshadowed successive Commonwealth meetings. Following violence and government-sponsored land-grabs from white farmers, Zimbabwe was suspended from membership in 2002 amid charges of abusing human rights.
In a report today for the Royal Commonwealth Society, the former Australian prime minister, Malcolm Fraser, says Zimbabwe is one of the organisation's "greatest failures".
"A number of Commonwealth leaders have been quietly involved in Zimbabwe over the years, but the Commonwealth itself could have been more influential and arguably did not marshal its resources early enough or adequately enough," Mr Fraser said.
"The slow road to recovery that we are now witnessing in Zimbabwe shows that it is high time for the Commonwealth to engage proactively with the Government of National Unity."