Duchess Catherine. Picture: Bang Showbiz
A royal aide has revealed that Catherine, Duchess of Cambridge is suffering from morning sickness.

The 35-year-old royal - who already has kids Prince George, four, and Princess Charlotte, two, with her husband Prince William - is currently pregnant with her third child, and after she made her first public appearance in six weeks at Buckingham Palace on Tuesday night, an aide offered an update on her current health situation.

The aide - who was speaking at the event on World Mental Health Day - told HELLO! magazine: "The Duchess' condition is improving but she is still suffering from hyperemesis gravidarum. She is delighted to be able to be here tonight."

Stephen Fry, the president of mental health charity Mind, also thanked the Duchess for making the effort to attend the reception at Buckingham Palace.

He said: "I do understand she's been having this problem more severely than many pregnant mothers and so we're very grateful (she's here)."

The actor also explained the need to make people more aware of mental health issues.

Read: Duchess Catherine's craving spicy food and candy

Stephen - who has been diagnosed with bipolar disorder - said that the event underlined the change in the public's thinking about mental health.

The comedian said: "When mental health is discussed here under these chandeliers in front of these portraits it does have a meaning it does show a change in our society's ability to look at problems like mental health."

Duchess Catherine and her husband have both played leading roles in the Heads Together initiative, which is designed to support people with mental health troubles.

And Stephen has hailed the work of the young royals.

He explained: "No one would realise if they hadn't done it, which makes it all the more remarkable that they have."