The Zimbabwe government said in a statement on Sunday that President Robert Mugabe had told the leaders of Jamaica, Nigeria and South Africa when they phoned him one after another on Sunday that Harare did not accept the Commonwealth's position and was leaving the group.
"Accordingly, Zimbabwe has withdrawn its membership from the Commonwealth with immediate effect," it said.
A divided Commonwealth extended sanctions against Zimbabwe on Sunday, but opened the way for a possible return to the 54-member group if Harare engages in reconciliation with the opposition.
Talks on Zimbabwe dominated a four-day Commonwealth summit in the Nigerian capital and caused the worst split in the club of mainly former British colonies since South Africa's apartheid in the 1990s.
The Zimbabwe government statement said the three leaders who had called Mugabe on Sunday to brief him on the Commonwealth's decision had tried to persuade him not to quit, but Mugabe was adamant there was no point for Harare to remain in the group.
"This is unacceptable. This is it - it's quits and quits it will be"
Zimbabwe government statement
"In response, President Mugabe indicated to each of the three leaders that the decision was unacceptable, as the Republic of Zimbabwe would settle for nothing short of its removal from the Commonwealth suspension and agenda," it said.
The statement said Mugabe had told them, "Anything that you agree on Zimbabwe which is short of this position, no matter how sweetly worded, means Zimbabwe is still a subject of the Commonwealth. This is unacceptable. This is it - it's quits and quits it will be."
Mugabe had also indicated that he would not be talking to any leader about the Commonwealth because Zimbabwe was no longer a member of the group, the statement said.
"On the suggested visit to Zimbabwe by leaders of the CHOGM committee, President Mugabe emphasised that Zimbabwe would receive and welcome them in a brotherly and friendly way but only as leaders of their respective nations, not as representatives of the Commonwealth," it added.
The 79-year-old leader, in power since independence in 1980, has sympathy from a small but powerful group of southern African nations which had lobbied for his country's re-admission at the biennial summit in the Nigerian capital Abuja.
Leaders tried to contain the Zimbabwe feud early in the conference by appointing the six-man task force, split roughly between Mugabe friends and foes.
But the move prolonged the dispute, and Mugabe's threats from Harare to withdraw raised the stakes further.
British PM Tony Blair (R) chats with Canadian PM Jean Chretien at the summit
Mugabe accused Britain and other "Anglo-Saxon" countries of punishing him for land reforms that have given white-owned farms to landless blacks.
His argument finds resonance with many other African leaders whose political lives started in the fight against British imperialism.
Many Africans suspect British demands for democratic reforms in Zimbabwe are just a cover for protecting white farmers and their land.
Blair said it was wrong to "muddle" that issue with questions of Zimbabwean democracy and rights violations.
"The vast majority of countries - black or white or Asian - are in favour of continuing the suspension because we can see that Zimbabwe is so clearly in breach of all the principles the Commonwealth stands for," he said in a television interview.
But thousands of miles away in Zimbabwe, Mugabe disparaged the Commonwealth as "a mere club" and likened it to George Orwell's classic political satire "'Animal Farm', where some members are more equal than others".