The World Development Movement reacts to the decision in this article from the Telegraph.
The group, which was part of last weekend's huge Put People First Alliance which held a rally in London, said the Foreign Office received a note from 10 Downing Street telling it to revoke the accreditation.
Benedict Southworth, the group's director, said: "I am outraged that we have apparently been banned at the last minute from attending the summit.
"We hope it is not what it appears to be – an attempt to stage-manage events and prevent voices of dissent and disagreement being heard.
"The only other government in the world that has banned the World Development Movement from attending a global summit is the Singapore government which has a track record of stifling voices of opposition."