In less than two weeks the fate of the people of Zimbabwe will be determined by the result of a run-off presidential election. If Robert Mugabe is allowed to steal that election the tragedy will be complete. The scale of the catastrophe that Mugabe has precipitated in his country is almost unimaginable. In just ten years, life expectancy has plummeted from 61 years to less than 36 - the lowest in the world. The economy has disintegrated - inflation by the official measure stood at 164,900 per cent in April, unemployment is more than 80 per cent; the shops are empty, the health service has collapsed, the school system no longer functions and millions of Zimbabweans have fled.

Amid the chaos and misery for ordinary Zimbabweans there exists a grotesque contrast. It is to be found in the ostentatious houses, newly built in the suburbs of Harare by Mugabe's party cronies and the military top brass; in the expensive cars that chauffeur the Zanu (PF) elite around the capital and the luxury foods available to those with access to foreign currency. But this grotesque contrast is most sinisterly apparent in the foreign currency miraculously found to arm and equip the forces that brutalise Mugabe's opponents, while public services and infrastructure crumble.

In view of the extreme circumstances facing Zimbabwe, I urged Gordon Brown two weeks ago to warn Mugabe that unless his Government met the basic minimum standards for a free and fair election on June 27 we would work with our allies in the region and the wider world to do the thing that his regime fears: cut off access to the foreign currency that keeps them in power. This step could be taken straight away by Britain using the powers of the Exchange Control Act 1947.

Since everything hinges on what happens in the coming days, a sharp and aggressive strategy with immediate consequences is justified and this is the only tool with sufficient force to secure the guarantees that we need now to ensure there is a fair election. We propose that its application should be reviewed weekly and be lifted immediately should the regime meet basic requirements for fair elections.

Blocking Zimbabwe's access to foreign currency would be a serious step and I do not propose it lightly. I know that many ordinary Zimbabweans rely on remittances from friends and relatives abroad. But access to foreign currency is what sustains Mugabe's brutal rule; blocking it is the only step that will have an impact on his regime because it would threaten its ability to function.

Since I raised this matter with the Prime Minister, the political situation in Zimbabwe has deteriorated even farther. Aid agencies have been banned from distributing desperately needed food, Morgan Tsvangirai, the opposition leader, has been detained five times and prohibited from holding rallies; more than 60 opposition supporters have been killed, and thousands have been beaten, intimidated and driven from their homes. Mugabe at the weekend said that he was willing “to go to war” if he lost. The Joint Operations Command, made up of the heads of the military and state security organisations, is already directing a violent campaign to “decompose” the Movement for Democratic Change.

Mr Brown said that he was willing to consider any measure that might secure a free and fair election, but I fear that in the end we will settle for nothing more than the usual hand-wringing and ritual condemnation.

The British Government has faced a difficult dilemma in tackling the Zimbabwe crisis. The Foreign Office has been understandably fearful that robust action against Mugabe's regime would play into his hands by discomforting our allies in southern Africa and by allowing him to characterise the MDC Opposition as stooges of Zimbabwe's “colonial oppressors”.

The Government's reticence may have been understandable while hope remained that Thabo Mbeki, the President of South Africa, would act decisively, but that hope faded long ago. In any event, anyone who has recently read the pages of the Zimbabwe Herald recently, or heard the broadcasts of the state radio or television channels, will know that the virulence of Mugabe's anti-British/anti-MDC rhetoric is already so extreme that he could not increase the level of vitriol even if he wished to.

Critics of the measures I have proposed argue that blocking foreign currency from entering the country would precipitate greater suffering. I do not underestimate the severe consequences.

The alternative, however, is to do nothing. That may spare us our moral qualms but it would not spare us the responsibility for the far greater disaster that will engulf Zimbabwe if Robert Mugabe is allowed to steal the election. The consequences for Zimbabwe's people of that outcome would be catastrophic beyond any imagining.

Nick Clegg is leader of the Liberal Democrats

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