The African Union chairman, Ghanaian President John Kufuor, has said that Zimbabwe's leader has a right to attend the EU-Africa summit in Portugal.
No-one has the right to exclude anyone says John Kufuor
UK Prime Minister Gordon Brown has said he will boycott the December summit if President Robert Mugabe is there.
"Africa is made up of 54 nations, sovereign states, and I don't think any of us has the right to exclude another," Mr Kufuor told the BBC.
The summit hosts, Portugal, said that all African countries would be invited.
Mr Mugabe faces a travel ban in Europe and his government is subject to European Union sanctions.
The BBC's world affairs correspondent Mark Doyle says AU chairman Mr Kufuor is the continent's senior voice.
And as the democratically elected leader of a stable Ghana, he is a close ally of the UK.
But like many other African presidents he sees Mr Mugabe as an historically important colleague - and objects to the idea of UK, a former colonial power, trying to say which Africans are welcome at the summit, our correspondent says.
Mr Kufuor said it was historic that Europe and Africa meet to look ahead especially to meet the challenges of globalisation.
"All the presidents of Africa were invited to this summit because we see it as a meeting of two continents, Europe and Africa," he said.
Mr Mugabe faces an EU travel ban and sanctions
"If we allow anyone - I wouldn't say even a nation, but an individual, whatever we think of him - to be a stumbling block then I say it's unfortunate."
Key former Portuguese colonies in Africa, including the large and oil-rich nation of Angola, made it clear to Portugal, a relatively small European state, that they wanted Mr Mugabe to attend.
The UK accuses Mr Mugabe of rigging elections and human rights abuses.
But the 83-year-old Zimbabwean president plays the old colonial card deftly in Africa and, with his invitation to Portugal, may now claim to have won another round against the UK, our correspondent says.