Mr Miliband, 39, one of the Prime Minister’s most-trusted lieutenants, has returned home with the baby boy, it emerged yesterday.

The couple would not have been allowed to adopt a British baby by the same method and were able to adopt the boy only because Mr Miliband’s wife, Louise Shackleton, holds dual British and American citizenship. Britain’s largest adoption and fostering charity has raised questions about the adoption and asked why the couple had not adopted a British child.

The Milibands refused to discuss the circumstances surrounding the adoption or to discuss whether there had been a financial arangement with the baby’s natural mother.

The child, named Isaac James Miliband, was born on December 13, weighing 7lb, before being taken from his natural mother and handed to the adoptive parents, who flew back to London on Christmas Eve.

Mr Miliband’s wife, a second violinist with the London Symphony Orchestra, was educated in America. The couple married in 1998, three years after meeting on a Rome to London flight.

The MP for South Shields announced the adoption in a statement that described the couple as “absolutely delighted” and “looking forward to all the joys and challenges of parenthood”.

“This is a wonderful end to the year for Louise and I, and our families,” he said.

Mr Miliband confirmed later that he and his wife had witnessed Isaac’s birth, saying that “anyone who has had a child will be aware of all the emotions we felt”.

He refused, however, to discuss the arrangement made with the birth mother or the couple’s reasons for not adopting a British child, saying that it was “a private affair”.

“Louise grew up in the US through her childhood and into her 20s. She is an American as well as a British citizen and she has adoption rights in America,” he said.

The vast majority of the 4,000 children adopted in Britain each year come from local authority care and it is estimated that only a few hundred of these are less than six months’ old. Childless couples who are approved for adopting a British baby can face a wait of several years and the Milibands’ case would not have been helped by the fact that Ms Shackleton is in her early 40s. Had she not held an American passport, the couple would not have been allowed to adopt Isaac unless they could satisfy the British Government that “the child cannot be cared for in any suitable manner in his own country” — unlikely in the case of a child born in the US.

An estimated 30,000 infants are adopted in America every year and the fees paid by adoptive parents for a healthy white baby can be as high as $40,000 (£20,760).

A spokesman for the British Association for Adoption and Fostering, said last night that, even if they had been approved for adoption, the Milibands’ would not have been allowed to leave a British hospital with a newly born baby, out of concern for the mother.

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