Prince Charles with Prince William. Picture: Bang Showbiz

Prince Charles is "so pleased" to have become a grandfather for the third time after his son Prince William welcomed a son into the world this week.

The 69-year-old royal became a third-time granddad when his son Prince William and daughter-in-law Catherine, Duchess of Cambridge, welcomed their son - who is yet to have a name - into the world on Monday, and he has now expressed his "joy" at the news.

The news was revealed in a joint statement from Charles and his wife Camilla, Duchess of Cornwall, which was tweeted from the Clarence House Twitter account.

It read: The Prince of Wales has released a statement following the birth of The Duke and Duchess of Cambridge's third child.

"HRH said: "We are both so pleased at the news. It is a great joy to have another grandchild, the only trouble is I don't know how I am going to keep up with them." (sic)"

News of the birth of the couple's third child was revealed on Twitter on Monday afternoon, in a statement from Kensington Palace.

It read: "Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 1101hrs.

"The baby weighs 8lbs 7oz.

"The Duke of Cambridge was present for the birth.

"Her Royal Highness and her child are both doing well. 

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

On Monday evening, 36-year-old Catherine - who also has four-year-old Prince George and two-year-old Princess Charlotte with Prince William - left St Mary's hospital in Paddington, London, and was pictured outside the ward with her 35-year-old spouse and their new arrival.

Meanwhile, Prince William thanked the medical staff who helped deliver his son.

A separate statement read: "Her Royal Highness The Duchess of Cambridge will be leaving St. Mary's Hospital this evening. The Duke and Duchess of Cambridge and their son will travel home to Kensington Palace.

"Their Royal Highnesses would like to thank all staff at the hospital for the care and treatment they have received. They would also like to thank everyone for their warm wishes."