The Prime Minister is boycotting the gathering in Lisbon because of the presence of Zimbabwe's Robert Mugabe, who is condemned by Britain for wide-ranging human rights abuses.

However Clare Short, also a former international development secretary, claimed that Lady Amos was only being sent in his stead because she is black.

"I'm afraid that there really isn't any other explanation," she told BBC Radio 4's The World at One.

"I don't see any reason to send a kind of pseudo minister and I think that it's not right to send her because she's black. I don't see any other reason for sending her."

Her comments were swiftly denounced by Foreign Secretary David Miliband, who said that Lady Amos knew the issues involved well and would be an effective representative for Britain.

"I think that is a bit insulting to Baroness Amos," he told The World at One.

"She is a former secretary of state for international development, she is a former leader of the House of Lords, she has got a lot of knowledge about Africa as a whole, not just Zimbabwe. I think she will be a very good advocate for the UK and also for the sort of relationship between the EU and Africa that we very much want to see."

Mr Miliband also defended Mr Brown's decision to stay away in the face of criticism from European Commission President Jose Manuel Barroso.

"It would have been absurd for the Prime Minister or myself to sit next to Robert Mugabe through a discussion of good governance and human rights and pretend that there wasn't absolute meltdown going on in Zimbabwe," he said.