Leaders of the European Union and the African Union will meet on Saturday and Sunday in Lisbon to discuss trade, migration and other issues.

But the meeting, the first to be held since 2000, has been dogged by controversy over inviting Zimbabwean President Robert Mugabe, who is widely blamed for running his country's economy into the ground and suppressing political opposition.

Some activists also have criticised the participation of leaders from Sudan, which has drawn international criticism for its alleged support of militias accused of atrocities in the restive Darfur region.

"No time has been set aside for formal or informal discussion (of the Zimbabwe and Darfur issues). What can we say of this political cowardice?" 17 African and European writers said in an open letter to African and European leaders.

Germany's Grass and South Africa's Gordimer were joined by Nigerian writer Wole Soyinka, also a Nobel laureate, and former Czech president Vaclav Havel, a playwright.

"Millions of Africans and Europeans would expect Zimbabwe and Darfur to be at the very top of the agenda. It is not too late," they said.

The letter was released by the non-governmental organisation Crisis Action. A spokesman for the group said it would be published in a number of African and European newspapers.

It was the latest salvo in a dispute that threatens to overshadow the summit, which the EU hopes will strengthen its economic and political ties with Africa at a time when China is making inroads on the impoverished continent.

The prospect that Mugabe, who has ruled Zimbabwe since independence in 1980, will attend has loomed over the meeting for several months.

British Prime Minister Gordon Brown has said he will boycott the event rather than share the stage with the veteran Zimbabwean leader. Czech Prime Minister Mirek Topolanek is also expected to stay away.

The 53-member AU has held firm in its demand that Mugabe be allowed to attend, and the majority of EU governments have backed its stand.

Germany and Portugal, which holds the rotating six-month EU presidency, are among those who have said they do not want the impasse to block the summit.

Mugabe, who sparked international outrage earlier this year when his police arrested and beat dozens of political opponents, became persona non grata in much of Europe after winning a 2002 election described as rigged by international observers.

Mugabe and more than 100 Zimbabwean officials are banned from travelling to EU nations under sanctions imposed that year.

Britain, which ruled Zimbabwe until independence in 1980, is expected to send a junior delegation to the summit.