UK Prime Minister Gordon Brown is boycotting an EU-Africa summit in Lisbon because it is being attended by Zimbabwe's president Robert Mugabe.
Mr Brown said he would not attend the summit if President Mugabe did
Mr Brown has said he will not sit at the same table as Mr Mugabe, because of his human rights record.
But European Commission President Jose Manuel Barroso attacked the move saying leaders should be prepared to meet people of whom they do not approve.
The UK has accused Mr Mugabe of human rights abuses and rigging elections.
Mr Mugabe faces a travel ban in Europe but he was allowed to attend the summit after African leaders threatened to stay away if he was not invited.
Mr Barroso criticised Mr Brown's decision to stay away from the summit, saying: "If you are an international leader then you are going to have to be prepared, on occasion, to meet some people your mother would not like you to meet.
"That is what we have to do from time to time."
And European Parliament president, Hans-Gert Poettering, said relations between Africa and the EU were more important than Mr Mugabe's presence at the meeting.
But former international development secretary Baroness Amos, who will be attending the summit in Mr Brown's place, told BBC Radio 4's Today programme it was right that the prime minister stood up for his principles.
"Every leader makes his or her own decision about who they will meet and the circumstances in which they meet them.
"There's no doubt that this summit would become a media circus if the prime minister of Britain were there with Robert Mugabe," said the Labour peer.
Mr Brown's decision to boycott the summit was backed by the Conservatives but shadow foreign secretary William Hague said Baroness Amos should make the British government's feelings plain to Mr Mugabe.
"It is a shameful episode for Europe that President Mugabe is to be feted in Lisbon," said Mr Hague.
"It is important that Baroness Amos, the minister representing Britain, lays his crimes bare before all those attending.
"The British people will want to know that these points have been made and that every leader attending the summit from Europe and Africa has had to take heed."
He added: "Mugabe should not go home without being made to feel deeply uncomfortable and those who welcome him should not go home without feeling ashamed."
Zimbabwe's official inflation is given at around 8,000% - the highest in the world and Mr Brown has said President Mugabe had to take full responsibility for the collapse of the country's economy.
The two-day Lisbon summit will include the launch of a new partnership between the EU and Africa to tackle issues such as development, good governance, peace, security, migration, energy and climate change.
The EU will also be attempting to draw up a number of new trade agreements with individual African countries and regional blocs.