It said it had "decided that The Gambia will never be a member of any neo-colonial institution and will never be a party to any institution that represents an extension of colonialism".
President Jammeh's action is a "disaster", says Gambian journalist Ebrima Sankareh
But Bakary Dabo, former vice president of The Gambia and chairman of the country's campaign for democratic change, said people there are generally "very happy" to be part of the Commonwealth.
He said the government had recently begun "picking up war against poorly specified enemies called 'western powers'".
Speaking on BBC Radio 4's Today programme, Mr Dabo said this rhetoric had "accelerated" in recent years and been used when President Jammeh gave a "rambling" speech at the United Nations General Assembly last month
Aids claim
Also at the UN, President Jammeh said homosexuality was one of the three "biggest threats to human existence".
He has also drawn international criticism for claiming he can cure Aids with a herbal body rub and bananas.
BBC Africa analyst Farouk Chothia said despite its image as an idyllic holiday destination, The Gambia, and its population of less than two million, were kept under tight control by its eccentric leader.
The latest decision was bound to come from him, our analyst added.
The Royal Commonwealth Society, an education charity which works in Commonwealth countries, said The Gambia's announcement was unexpected and appeared to be undemocratic.
Society director Michael Lake said President Jammeh had made the decision "without consulting The Gambia's people" and the country's withdrawal would be "a loss felt by both its people and the wider Commonwealth network".
He added: "Far from being a 'neo-colonial institution', the modern Commonwealth operates on a consensus model and its voluntary membership is predicated primarily on a country's commitment to upholding shared values and principles.”
The last time a nation left the Commonwealth was in 2003, when Zimbabwe withdrew.
The Queen, who is 87, is the head of the Commonwealth, which holds its next heads of government meeting in Colombo, Sri Lanka, next month.
However, the gathering will be the first that the Queen has not attended since 1971. She will send her son, the Prince of Wales instead. Buckingham Palace said she would make fewer overseas trips because of her age.
Three African countries have joined the Commonwealth in recent years.
Rwanda was admitted in 2009 after applying for membership the previous year, while Cameroon and Mozambique became members in 1995.
GAMBIA:
- Official name is The Gambia to distinguish it from the River Gambia
- One of Africa's smallest countries, it is surrounded on three sides by Senegal
- Its borders are said to have been set in the 19th Century by the distance a naval cannon could fire from a British gunboat on the river
- Tourism, fishing and peanuts are major industries
- Governed since 1994 by President Yahya Jammeh, who seized power aged 29
- Last year he vowed to stay in power for "a billion years", if God willed it
- Claims he can cure Aids and infertility with herbal concoctions
- Warned in 2008 that gay people would be beheaded
bbc.co.uk
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