The comments from Mark Malloch Brown followed a similar shift of tone from Washington, which has dropped its public demand for President Robert Mugabe to step down since he and rival Morgan Tsvangirai agreed on the power-sharing government.
Malloch Brown said he had been convinced by African leaders at a summit in the Ethiopian capital that the government between Mugabe and opposition leader Tsvangirai must be given a chance.
"I think the one message I’ve got loud and clear from this summit, and I’m very sympathetic to it, is we’ve got to give this a go, we’ve got to all do our best to support it, because the needs of Zimbabweans are so overwhelming," Malloch Brown told BBC radio in an interview from Addis Ababa.
"We’re sceptical but we’ve got to try and help this work," he said, saying Britain and others would be generous donors if the agreement succeeded.
The new government, with Tsvangirai as prime minister, is due to be sworn in by February 13, although the opposition MDC accused Mugabe’s ZANU-PF on Tuesday of backtracking on the agreement by delaying discussions on contentious issues.
Former colonial power Britain has been one of the fiercest critics of Mugabe, accusing him of destroying the economy of the formerly prosperous country and using militias to violently suppress opposition. The veteran Zimbabwean leader blames the crisis on Western sanctions.
Zimbabwe suffers the world’s highest inflation rate, officially put at 231 million percent, and acute shortages of food, fuel and foreign exchange.
Malloch Brown’s remarks suggested African leaders may have persuaded Western powers to take a softer line over Zimbabwe while the power-sharing government starts work.
The new U.S. administration of President Barack Obama toned down rhetoric against Mugabe on Monday in a significant change from the previous Bush administration, which had intensified calls for the Zimbabwean leader to quit.
Analysts say Western rhetoric against Mugabe is often counter-productive in Africa, feeding his allegations that Britain and other powers are plotting to overthrow him.
Malloch Brown made clear, however, that Britain would not drop sanctions against Mugabe and his entourage until it had seen whether they were making a real commitment to power-sharing.
"We really hope this time it is different for the sake of the people of Zimbabwe and we will work as though it is different, but we are not going to completely put away our stick, if you like, until we’re convinced it is.
"That doesn’t mean we’re not going to put carrots on the table now, we are," he said.
He said a serious and credible economic reform programme must be put in place in Zimbabwe. "As that comes into place, you’ll find Britain and others being very generous donors."
The European Union this week also welcomed the deal and expressed hope the new government would repeal repressive legislation and create conditions for economic recovery.
The MDC (Movement for Democratic Change) in a statement accused Mugabe’s party of trying to scuttle the power-sharing deal by dodging discussions on outstanding issues. "There is no wish to consummate an inclusive government," it said.
The unity government may be a step towards saving a once prosperous country where over half of the people now need food aid and a cholera epidemic has killed 3,229 people and infected 62,909 others — Africa’s deadliest outbreak in 15 years.