Sofia is third-in-line to the Spanish throne, after her father and sister, Princess Leonor, born in 2005.

The birth of another princess eases immediate pressure for Spain to reform its constitution, which gives future male siblings precedence to the throne.

Spain's government says it wants to change the law so that females have the same succession rights as males.

However, this would need the support of two-thirds of both houses of parliament and require parliament to be dissolved and new elections called.

'Really happy'

Princess Sofia was born by Caesarean section at Madrid's Ruber International Clinic on Sunday and weighed 7lb 3oz (3.3kg), doctors said.

Following the birth, Princess Letizia and the newborn were said to be "in perfect medical condition".

Prince Felipe, who was present at the birth, said he was delighted with the arrival of a new daughter and said she looked a lot like her sister.

"We are very happy. I truly didn't know how I was going to react to the birth of another baby girl, but I think that similarly to the previous birth, here experience counts and one loves each child just the same.

"We are really happy with our newborn." He announced that she would be named after her grandmother, Queen Sofia.

The happy event contrasts sharply with Princess Letizia's grief after her sister, Erika Ortiz, died in an apparent suicide two months ago, following the end of a relationship.

Prince Felipe, 39, married Letizia Ortiz, 34, a former television news presenter, in May 2004. She gave birth to Princess Leonor in October 2005.