The infant, 13-month-old David Banda, was accompanied by one of Madonna's bodyguards on his flight. They were believed to be en route to the South African city of Johannesburg.

Jonathan Clayton, Johannesburg Correspondent of The Times, said that if the reports were correct that it was likely that the baby would continue his journey to the UK onboard a scheduled flight.

"We are assuming that the private plane is only flying locally and won't fly all the way to Britain. There are five flights scheduled to depart for London today. The assumption is that the baby will fly on a commercial flight to Britain," he said.

Mr Clayton said the adoption and subsequent flight today was a "legal nightmare" and it was not yet clear how the child had managed to leave the country without a passport. Mr Clayton said if Madonna had already fully adopted the baby there is a possibility that it could travel on her passport.

But it is still unclear if the singer will be able to bring the child into the UK, where she spends much of her time with her husband, the British film director Guy Ritchie.

Legal experts say that as Malawi is not designated as an approved country under the Hague Convention on inter-country adoption, any adoption formalised there will have no status in the UK. That means that the singer will have to go through a 'readoption" process that could take up to two years.

Human rights groups had planned to file an application today asking the court to block the adoption, which they say is illegal. Malawian law prohibits adoptions by non-residents, but officials have granted an exemption for the Ritchies.

The Eye of the Child, the leading child advocacy group in Malawi, said that the request for an injunction would be filed in a magistrate's court in the capital Lilongwe on behalf of about five dozen non-governmental organizations

Boniface Mandere, a spokesman for the children's rights group said: "They [government] haven't followed the law. What has happened is a shortcut."

The couple arrived in Malawi on October 4 for a humanitarian trip and left last Friday.

A Home Office spokeswoman said that all people who arrive in the UK should have a valid passport or visa and that babies who arrive in this country without the correct documents are dealt with on a case-by-case basis. She said that in the UK babies need their own passport but that she could not comment on the criteria of other nations' passports.

While in the impoverished nation the world's highest earning female singer spent most of her time visiting orphanages as part of a campaign to publicise the plight of some 900,000 orphans.

She has pledged to donate about $3 million to the campaign to help these children, many of whom are infected with HIV, through her Raising Malawi charity.

A final court decision on the adoption was expected within two years, after officials have had a chance to monitor how the child relates to his new environment in the United States and Britain where Madonna and Mr Ritchie have homes, according to a senior government official.

The couple already have a son, Rocco, 5, and the singer also has a daughter, Lourdes, 9.

The baby's father, Yohame Banda, said last week that he was pleased the celebrity couple wanted to adopt his son, whose mother died soon after giving birth to him. He said Madonna had promised him to bring his son back to visit him when he is older.

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