Pippa Middleton comforting Duchess Catherine through morning sickness

Pippa Middleton (Bang)

Pippa Middleton has been spending lots of time with her sister Duchess Catherine while she is suffering with morning sickness.

The 35-year-old beauty has been convalescing at Kensington Palace ever since it was announced she is expecting her third child with Prince William on Monday, and her younger sibling has been by her side most of the time comforting her, an insider told Us Weekly magazine.

Catherine is once again suffering from Hyperemesis Gravidarum - a debilitating form of serious morning sickness - but William has assured she is on the mend and once she is back on her feet they will be celebrating their baby news.

The Duke of Cambridge - who already has Prince George, four, and two-year-old Princess Charlotte with his spouse - said: "We need Catherine to get over this first bit and then we can start celebrating.

It's always a bit anxious to start with, but she's well. There's not much sleep going on at the moment."

The British royal couple surprised the world with their announcement earlier this week.

Kensington Palace said in a statement: "Their Royal Highnesses the Duke and Duchess of Cambridge are very pleased to announce that the Duchess of Cambridge is expecting their third child.

"The Queen and both their families are delighted with the news."

Catherine was forced to cancel a planned official engagement in London later in the day because of her morning sickness, which she also experienced when she was carrying both George and Charlotte.

The statement concluded: "As with her previous two pregnancies, the Duchess is suffering from Hyperemisis Gravidarum. Her Royal Highness will no longer carry out her planned engagement at the Hornsey Road Children's Centre in London today. The Duchess is being cared for at Kensington Palace."

William's younger brother Prince Harry was one of the first to congratulate the couple on their baby news.

The 32-year-old royal told reporters: "[It's] fantastic, great. Very, very happy for them."

And he also told well-wishers that Catherine is doing "OK" despite being poorly.

He said: "I haven't seen her for a while but I think I she's OK."