The Duke and Duchess of Cambridge are "absolutely delighted" that Kate's sister, Pippa Middleton, is to wed her financier boyfriend, James Matthews, Kensington Palace has said.
William and Kate shared their joy at the news, as did Pippa's father, Michael Middleton, who declared they are a "wonderful couple" and that he and wife Carole are "absolutely thrilled".
Pippa and her future husband will wed in 2017 - in what is set to be one of the most talked about society weddings since William and Kate tied the knot five years ago.
A spokesman for the Cambridges said: "The Duke and Duchess of Cambridge are absolutely delighted with the news."
Mr Middleton said in a statement released to the Press Association: "Carole and I are absolutely thrilled with the news. They make a wonderful couple and we wish them every happiness together."
Pippa and Mr Matthews also shared their happiness in their own brief joint statement.
"Miss Pippa Middleton and Mr James Matthews are delighted to announce they became engaged on Sunday 17th July and plan to marry next year," they said.
The smiling bride-to-be showed off her sparkling diamond engagement ring as she was pictured leaving her house.
Hedge fund boss Mr Matthews got down on bended knee during a trip to the Lake District at the weekend to propose to Pippa.
The couple have been dating for less than a year, but had moved in together and were last week seen at Wimbledon enjoying the tennis. A thrilled Pippa was said to have been totally surprised by the proposal.
On Monday night, Pippa's ring finger was bare when she stepped out at the Frost Summer Party Fundraiser which raises money for the British Heart Foundation's Miles Frost Fund, but on Tuesday morning she was seen wearing the large diamond, surrounded by smaller diamonds.
Pippa leapt into the public consciousness in 2011 when she was her sister's bridesmaid in a figure-hugging Alexander McQueen dress at the royal wedding.
Royal watchers are expecting Prince George and Princess Charlotte to play an important role - as page boy and flower girl, while Kate looks set to be her sister's matron of honour.
Mr Matthews is the elder brother of former Made In Chelsea star Spencer, who said in a statement: "I'm delighted for both of them."