Kate to Join Queen and Camilla at a London Tea

By Simon Perry

02/13/2012 at 07:35 AM EST

Call it tea with Kate and the in-laws.

The Duchess of Cambridge will join her grandmother-in-law, Queen Elizabeth, and her stepmother-in-law, Camilla, the Duchess of Cornwall, at an official engagement at Piccadilly's flagship Fortnum and Mason's store on March 1, as "three royal ladies have tea," a royal source tells PEOPLE.

The tea party with the trio will serve as the centerpiece of an event to mark the regeneration of the London area of Piccadilly and be the first of two outings for Kate while William is serving as a Search and Rescue helicopter pilot in the Falkland Islands.

The second event, on March 8, will find Kate joining the Queen and Prince Philip on a field trip to Leicester.

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Until then, Kate is scheduled to visit Liverpool Tuesday, to pay her respects at an alcohol-free bar connected to the charity Action on Addiction and a children's hospital.