The Duchess of Cambridge is pregnant with her second child.

Kensington Palace said the Queen and members of both families were delighted with the news.

The palace said that – as when she was pregnant with Prince George – the duchess was suffering from very acute morning sickness and was being treated by doctors at Kensington Palace.

The baby will become fourth in line to the throne, moving Prince Harry further down the line of succession.

Prime Minister David Cameron, Labour leader Ed Miliband and the Archbishop of Canterbury were among those to offer their congratulations to the couple.

BBC royal correspondent Peter Hunt said that for a second time Prince William and his wife have had to announce a pregnancy before the duchess had passed the 12-week stage.

Our correspondent added that said this pregnancy was likely to generate excitement but was lacking the constitutional significance of Prince George’s birth – as things stand the unborn boy or girl is not destined to be monarch.

Clarence House said: “Their Royal Highnesses the Duke and Duchess of Cambridge are very pleased to announce that the Duchess of Cambridge is expecting their second child.”

The duchess’s morning sickness meant that she would no longer accompany Prince William on their planned engagement in Oxford on Monday, Clarence House said.

Catherine had been due to join William in formally opening a £21m China study centre at Oxford University.

The sickness – called hyperemesis gravidarum – is a condition that may require supplementary hydration, medication and nutrients.

The duchess had been due to carry out her first solo overseas tour in Malta later this month.

The baby announcement came two months after the couple’s first child, Prince George, had his first birthday.

Unlike when she was pregnant with George, the duchess is being treated at Kensington Palace and not at a hospital.

The first pregnancy was revealed when the duchess was just a few weeks pregnant after she was admitted to hospital suffering from severe morning sickness in December 2012.

Hyperemesis gravidarum, which affects 3.5 per 1,000 pregnancies, causes severe vomiting and can lead to dehydration, weight loss and a build-up of toxins in the blood or urine, called ketosis.