He has been accused of dithering over foreign policy, labelled a “hermit prime minister” by one critic and is eclipsed on the world stage by more forceful leaders. Yet Gordon Brown may be in a position finally to redeem himself by playing a pivotal role in resolving one of the toughest crises to face Africa in the past decade.

The Prime Minister sets off on a four-day trip to America next week, where he will attend a meeting at the United Nations, visit President Bush and deliver a speech in Boston.

Unlike previous prime ministerial missions to America the key event could be the UN fixture rather than his date at the White House. It is here, in the company of President Mbeki of South Africa, that senior British officials hope progress may be made to hasten the end of the rule of Robert Mugabe in Zimbabwe and begin the country’s long path to reconstruction.

Experts caution that dislodging the 84-year-old dictator and orchestrating a peaceful handover of power is a huge challenge and one that notably eluded Tony Blair and a succession of British foreign secretaries.

But there is a real sense in Whitehall that Mr Mugabe has at most weeks or months left in office and that Britain can play a decisive role behind the scenes to ensure that the great survivor of southern African politics does not wriggle off the hook one last time.

Much will depend on high-stakes diplomacy this weekend.

Lord Malloch-Brown, the Foreign Office minister responsible for Africa, is paying a discreet visit to Beijing where he will try to persuade China to drop its support for the Mugabe regime.

In the past the Chinese, who have a veto on the UN Security Council, have prevented Zimbabwe from being raised. They have insisted that the crisis is an internal matter and does not constitute a “violation of international peace and security”, the prerequisite for UN Security Council action. Already under growing international pressure over Tibet, Darfur and Burma, the calculation is that China is unlikely to take a stand over the crumbling regime of Mr Mugabe.

The next challenge is to overcome the timid behaviour of Zimbabwe’s African neighbours. This is being made easier by the refusal by Mr Mugabe to attend an emergency summit on Zimbabwe being held in Zambia today by the 14-member Southern African Development Community.

The African leaders wanted an explanation about the failure of the Zimbabwean authorities to release the results of the presidential election held two weeks ago or to set a date for a run-off. Without the presence of Mr Mugabe a tougher position is likely to be adopted. His nonappearance will also reinforce the impression that he is no longer in control of the country, which some suspect may already have passed into the hands of the security elite around him.

The recent violence, combined with the collapsing Zimbabwean economy, its record inflation and the flight of millions of its citizens to neighbouring states, will give Mr Brown the opportunity to argue that Zimbabwe requires the attention of the UN Security Council urgently.

Getting the issue before the council could be a hugely significant first step. The Security Council has the power to enforce international law, authorise the use of force and dispatch peace-keeping troops.

Although Britain has succeeded before in pushing Zimbabwe on to the agenda, to discuss brutal episodes in the rule of Mr Mugabe, no one will be under any illusion that this time the council is being brought in to help to administer the death rites to his regime.

But Mr Brown will need guile, persuasion and passion to carry the day when Zimbabwe is expected to be discussed in the margins of the Security Council summit on African Union-UN cooperation on Wednesday.

He will be accompanying President Mbeki, whose country currently holds the rotating presidency of the Security Council. Although the head of a regional superpower, Mr Mbeki has failed to confront Mr Mugabe over his destructive policies. The “quiet diplomacy” of the South African leader has notably failed to halt the destruction of the Zimbabwean economy.

After two hours of talks in London last weekend Mr Mbeki did give Mr Brown a commitment that he would support action in the UN if no progress had been made to end the crisis.

Even with the support of South Africa other Security Council members could also block moves to draw up a tough statement against Zimbabwe because the wording of any text requires unanimity.

Libya, formerly a close ally of the regime, which has pursued eccentric policies in Africa, could block any critical statement against Harare. The Libyans and others may also resent Zimbabwe’s former colonial power taking such a strong position in deciding its future.

Mr Brown will have to persuade world leaders that the debate over Mr Mugabe and his failed reelection attempt is over. What matters now is the future stability of Zimbabwe and the commitment of the international community to help the peaceful transfer of power and the multibillion-pound effort needed to rebuild the shattered country.

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