The Cornish language has been branded "extinct" by linguistic experts, sparking protests from speakers.
Thirty linguists worked on Atlas of the World's Languages in Danger, compiled by Unesco, the cultural section of the United Nations.
Cornish is believed to have died out as a first language in 1777.
But the Cornish Language Partnership says the number of speakers has risen in the past 20 years and there should be a section for revitalised languages.
Unesco's Atlas of the World's Languages in Danger features about 2,500 dialects. There are thought to be about 300 fluent speakers of Cornish.
But Jenefer Lowe, development manager of the Cornish Language Partnership, said there were thousands who had a "smattering" of the language.
"Saying Cornish is extinct implies there are no speakers and the language is dead, which it isn't," she said.
"Unesco's study doesn't take into account languages which have growing numbers of speakers and in the past 20 years the revival of Cornish has really gathered momentum."
Last year the partnership agreed a single written form of Cornish which brought together several different forms of the language.
Mrs Lowe said: "There's no category for a language that is revitalised and revived.
"What they need to do is add a category.
"It should be recognised that languages do revive and it's a fluid state."
Christopher Moseley, an Australian linguist and editor-in-chief of the atlas, told BBC News he would consider a new classification.
He said: "I have always been optimistic about Cornish and Manx.
"There is a groundswell of interest in them, although the number of speakers is small.
"Perhaps in the next edition we shall have a 'being revived' category.
"[Cornish] is among a group of languages that turned out not to be extinct but merely sleeping."
