The Duchess of Cambridge is expecting a baby, St James's Palace has announced.

Members of the Royal Family and the duchess's family, the Middletons, are said to be delighted.

A spokesman said the duchess has been admitted to King Edward VII Hospital in central London with acute morning sickness and is expected to stay for several days.

The royal baby will be born third in line to the throne, after Prince Charles and Prince William.

He or she will one day be head of the armed forces, supreme governor of the Church of England and subsequently head of state of 16 countries.

Catherine and William were married at Westminster Abbey in April 2011.

The duchess was last seen in public on Friday when she visited her old school St Andrew's School in Pangbourne in Berkshire.

In a statement, St James's Palace said: "Their Royal Highnesses the Duke and Duchess of Cambridge are very pleased to announce that the Duchess of Cambridge is expecting a baby.

"The Queen, the Duke of Edinburgh, the Prince of Wales, the Duchess of Cornwall and Prince Harry and members of both families are delighted with the news."

It said the duchess was being treated in hospital suffering from hyperemesis gravidarum, which requires supplementary hydration and nutrients.

"As the pregnancy is in its very early stages, Her Royal Highness is expected to stay in hospital for several days and will require a period of rest thereafter," it added.

St James's Palace refused to be drawn on when the royal couple became aware of the pregnancy, only saying "recently".

But it is understood that the pregnancy has not passed the 12 week point and the palace announcement was prompted by the Duchess's medical condition.

Asked about having children in an interview after their engagement, William said: "I think we'll take it one step at a time. We'll get over the marriage thing first and then maybe look at the kids, but obviously we want a family."

Prime Minister David Cameron wrote on Twitter that he was "delighted by the news", adding: "They will make wonderful parents."

Labour leader Ed Miliband tweeted: "Fantastic news for Kate, William and the country. A royal baby is something the whole nation will celebrate."

Royal historian Hugo Vickers said: "It's fantastic that there is another generation of the House of Windsor coming along in the reign of the present Queen so she will see a great-grandchild who will one day sit on the throne.

"We will be watching this baby from the moment it's born, and if we all live long enough until the moment it becomes king or queen. And that's the great advantage, I think, of hereditary monarchy."

© BBC