"I am deeply grateful to all those in Malawi who helped make this possible," she wrote.

The singer also appealed for privacy from the media during "this transitional time".

She got permission for the adoptions by a court in Malawi on Tuesday.

She already has two children from the African country - David, adopted in 2006, and Mercy, adopted in 2009.

Less than two weeks ago, during a charity visit to Malawi, Madonna denied reports she had applied to adopt any more children.

But court papers on Wednesday revealed the singer felt duty-bound to adopt the four-year-old girls, named Esther and Stella, from an orphanage after discovering information about their backgrounds through her charity work.

Their mother died soon after giving birth, the papers show, while their father married a different woman, leaving only their grandparents who were overwhelmed by the prospect of having to bring up the twins and their siblings.

Judge Fiona Mwale said the singer was able to supply a loving home life for the twins in "a luxurious, spacious and comfortable abode in an affluent neighbourhood" of the US.

She said that although the 58-year-old performer was "above the age normally considered within the ranges of parenting", she had supplied medical evidence to support her argument that she was in good health.

"Her charity work brings her into contact with orphanages... and it was as a result of this contact that she felt compelled to fill a gap in their lives and open up her home to them," the judge said.