The couple confirmed rumors in a statement to Britain's Press Association, saying they planned to marry in 2017. The statement came as Middleton appeared in photographs with a diamond engagement ring on her left hand as she left her London home.
Matthews got down on one knee while they were visiting the picturesque Lake District and proposed to the 32-year-old Middleton on Sunday. British media said they have been dating less than a year.
"They make a wonderful couple and we wish them every happiness together," said her father, Michael Middleton.
Middleton played a prominent role in the 2011 royal wedding of her older sister Kate and Prince William at Westminster Abbey in London, a wedding seen live by a worldwide television audience.