Kate, 32, will leave baby Prince George at home and head to Malta in September for a two-day visit.
“This is a seminal moment for her, when she will be showing herself to be stateswoman in her own right,” a palace source tells PEOPLE.
The Duchess of Cambridge was asked by the Queen, 88, to represent the royal family at the Mediterranean island’s 50th anniversary of independence celebration, which involves the kind of royal engagements that she would normally do together with Prince William.
Kate will be calling on Malta’s president, Marie Louise Coleiro Preca, and is also giving a speech, which insiders say she is “nervous” about.
Kate has made two speeches on foreign soil before, while touring Malaysia in 2012 and in Australia this year, but this is her first speech on behalf of the Queen.
On her September 20-21 visit, she will attend church and take a tour across the harbor. And at the national library she is set to view historical documents, including letters from King Henry VIII and George II.
And the Queen will be watching closely as Malta holds a special place in her heart – she and husband Prince Philip lived there for a few years during the early years of their marriage when he was a naval officer (and before she became the Queen).