The royal couple will tie the knot at Westminster Abbey in London followed by a meal hosted by Queen Elizabeth at Buckingham Palace and the 'Starry Eyed' singer has been invited to provide the after-dinner entertainment at an intimate gathering for 300 people hosted by William's father Prince Charles.
William and Kate reportedly chose the pop star because they believe her music will appeal to guests of all ages.
A source said: "William and Kate are huge fans of Ellie and were keen to sign her up.
"Her music goes down well with the young ones, plus a few of the older guests will be nodding their heads along.
"They really wanted a top British talent and the couple are delighted Ellie agreed to perform. She can't wait, although she's a tad nervous."
William, 28 and Kate, 29, hope Ellie will serenade them with her cover of 'Your Song' by Sir Elton John - who will attend the nuptials with his partner David Furnish - and the excited singer is said to be "honoured" with the invitation.
An insider told the Daily Mail newspaper: "Ellie was knocked off her feet when she received the invitation to perform for Prince William and Kate - and was so hugely honoured when they said they were both big fans of her music.
"She still can't quite believe that she will perform for the royal newlyweds, all the young royal guests and their friends, it hasn't sunk in yet."
Ellie, 24, will be accompanied by her boyfriend, DJ Greg James.