Kate, 41, and Sophie, 58, who share a close bond, as well as similar taste in style (see the clip below), will attend the Commonwealth Day service at Westminster Abbey on Monday 13 March.
WATCH: Princess Kate twins with Countess Sophie at Buckingham Palace reception
Kate and Sophie at the Christmas carol concert last December
Monday will mark the King and Queen Consort's first Commonwealth service of their reign, with Charles set to deliver a message marking the occasion from the Great Pulpit in the church where he will be crowned in two months' time.
Afterwards, Charles, Camilla and the senior royals will entertain the Commonwealth secretary-general, high commissioners, foreign affairs ministers and other members of the Commonwealth community at a Buckingham Palace reception.
Charles is now head of the Commonwealth, following in the late Queen's footsteps after she successfully lobbied for him to take on the role.
A Commonwealth flag for peace will be carried in the procession of Commonwealth members' flags to mark 2023 as Commonwealth Year of Peace.
Charles, Camilla, William and Kate at the Commonwealth Day service 2022
The service will include musical performances from saxophonist Yolanda Brown, West End stars Roshani Abbey and Nuwan Hugh Perera, and the all-female Amalgamation Choir, from Cyprus.
Guests of honour among the 2,000-strong congregation will include the Commonwealth secretary-general, the prime minister of Samoa, high commissioners, senior politicians and dignitaries from across the UK and the Commonwealth.
Also attending will be athletes from the home nations who competed at the Commonwealth Games in Birmingham last summer, as well as faith leaders, and more than 750 schoolchildren and young people from throughout the UK.
The service will be broadcast live on BBC One.
LISTEN: Kate's royal tour fashion choices revealed!
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