Millions of people around the world are set to tune in to watch Prince Harry and Meghan carry out their final public engagement before they step back from being senior royals. 

The couple will be embarking on a new chapter in North America, but first, they will join the Queen, Prince Charles and his wife Camilla at the annual Commonwealth Day service at Westminster Abbey in London next week.

Since shunning the spotlight on the royal family with their decision to quit royal life in January, the Duke and Duchess of Sussex are yet to be seen publicly with their royal counterparts, further highlighting the significance of the event. 

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Meghan and Harry shared photos on Instagram to wish Welsh followers a Happy St Davids Day on March 3. Photo: Instagram

Also present at the service on March 9 will be Prince William and wife Kate.

International boxing champion and Olympic gold medallist Anthony Joshua will deliver a reflection, while singers Alexandra Burke and Craig David will perform.

Harry and Meghan are expected to join the rest of the royal family greeting service participants.

That will be Harry and Meghan's final royal duty before stepping away from the monarchy completely on March 31 as they attempt to make a life in Canada away from the UK press. 

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The Duke and Duchess of Cambridge with Harry and Meghan at 2019's Commonwealth Day service. Photo: Getty

Guests of honour among the 2000-strong congregation will include British Prime Minister Boris Johnson, the Commonwealth secretary-general Baroness Scotland, high commissioners, ambassadors, faith leaders and more than 800 schoolchildren and young people.

Tradition dictates that sixth-in-line Harry sits with Meghan in the second row of seats behind the Queen, Charles, Camilla, William and Kate during the service.

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Meghan meets a young fan in Cardiff in 2018. The couple shared photos this week to mark St Davids Day. Photo: Instagram

The Queen and the royal family will meet people involved in the service and walk past the Commonwealth flag bearers as they leave.

Prince Andrew will be absent this year, having stepped down from public life following a disastrous television interview over his friendship with convicted sex offender Jeffrey Epstein.

-with AAP