Prime Minister Gordon Brown has said he would boycott a summit of African and EU leaders in December if Zimbabwe President Robert Mugabe attends.
Gordon Brown says it is "him or me" at the EU-AU summit
Mr Brown says Mr Mugabe's presence at the conference in Lisbon will "divert attention" from important issues such as poverty, climate change and health.
Zimbabwe was facing "a tragedy that requires the whole of the world to speak up and also to act", he told ITN.
But a Zimbabwean minister warned the UK should not pressure the EU in this way.
"If the European Union agrees to be dictated to by an individual member state like Britain, a former colony, and brings its colonial machinations on Zimbabwe to the EU, that would be unfortunate," Sikhanyiso Ndlovu, the minister for information, told the BBC.
He stressed that President Mugabe was a head of state, and a member of the African Union, and would attend the summit in Portugal if invited.
However, no invitation has yet been sent to Mr Mugabe, according to senior sources in the Portuguese government.
Foreign Secretary David Miliband told the BBC World Service that Mr Brown's view was the right one.
"An EU-Africa summit can only become a media circus if it's dominated by Robert Mugabe sitting next to Gordon Brown," he said.
"We don't think that it's right to be discussing good governance, human rights, economic development as if there isn't the tragedy unfolding in Zimbabwe that is unfolding at the moment, unfolding as a direct result of the policies being pursued by Mr Mugabe.
"That's why I think it was right for Gordon Brown, who has been absolutely clear in all of his foreign policy and domestic policy decisions, that he will be straight about those intentions, to say that if Robert Mugabe comes, then we won't attend."
The BBC's Europe editor, Mark Mardell, said he understood diplomats were being "very active in trying to find a compromise".
This could involve inviting another Zimbabwean representative, such as a junior member of the government or a civil servant, so that Mr Brown could attend, our correspondent said.
Portugal, which holds the rotating EU presidency, is keen to invite every African leader to be at the summit on 8 and 9 December, but may let the African Union decide who should attend.
Mr Brown described the summit as a "serious opportunity" to forge stronger partnerships between Africa and the EU.
"I applaud the prime minister of Portugal for what he is trying to do to build stronger relations between Europe and Africa. This is a summit that is necessary for Africa's sake.
"But of course it would be totally inappropriate for me to be there if President Mugabe [is]."
Denominations of bank notes have risen owing to levels of inflation
He went on: "Four million people have left the country. Four million people on food aid because of famine by Christmas, 80% unemployment, life expectancy at 37."
He said humanitarian aid was being stepped up and promised to press the UN Security Council for an envoy to "look and report on the situation".
Earlier the prime minister told the Independent that President Mugabe's presence "would undermine the summit, divert attention from the important issues that need to be resolved".
He added that Britain had a responsibility to the people of Zimbabwe, who found themselves in an "appalling" situation.
In order to allow Mr Mugabe to attend the conference, EU member states would have to convene before the summit and agree to lift the travel ban currently imposed on him.
But Mr Brown is urging EU leaders to keep it in place, saying there was "no freedom in Zimbabwe" and "widespread torture and mass intimidation of the political opposition".