“They both keep their roles as President and Vice-President of the Queen’s Commonwealth Trust.
“That is hoped to be a meaningful final engagement for the couple - their last ever as senior royals.”
Buckingham Palace has refused to confirm who will attend the Commonwealth Day service.
They said: “Members of the royal family have been invited to attend the Commonwealth Day service and the members attending will be announced nearer the time in the usual way."
Last week, details emerged that the Sussexes have sacked all 15 of their staff and closed their Buckingham Palace office.
The move comes after the couple announced their plan to step down as senior members of the royal family back in January, and is the surest sign that they are unlikely to return to the UK to live after moving to North America.
The Duke and Duchess of Sussex broke the news to their team in person in January, and while one or two may be absorbed back into the royal household, according to Daily Mail, most are now negotiating redundancy packages.
A source told the publication: "Given their decision to step back, an office at Buckingham Palace is no longer needed. While the details are still being finalised and efforts are being made to redeploy people within the royal household, unfortunately there will be some redundancies."
Those who have been ousted by the Sussexes include the couple's newly appointed private secretary, Fiona McIlwham and their hugely experienced communications chief Sara Latham, as well as their press officers, their social media expert and other members of their communications team.
The source insisted most accept their fate.
"The Duke and Duchess have a small team, less than 15 people. The team are very loyal to the Sussexes and understand and respect the decision they have taken," the source said.
"They are all close and supporting each other. The team are busy helping to set their Royal Highnesses up for the future and working on a series of final engagements."
Harry and Meghan shocked the world on January 6 when they announced their decision to quit, and they left the royal family in a state of turmoil.
In a statement they said: "After many months of reflection and internal discussions, we have chosen to make a transition this year in starting to carve out a progressive new role within this institution."
They continued: "We intend to step back as 'senior' members of the Royal Family and work to become financially independent, while continuing to fully support Her Majesty The Queen. It is with your encouragement, particularly over the last few years, that we feel prepared to make this adjustment.
"We now plan to balance our time between the United Kingdom and North America, continuing to honour our duty to The Queen, the Commonwealth, and our patronages. This geographic balance will enable us to raise our son with an appreciation for the royal tradition into which he was born, while also providing our family with the space to focus on the next chapter, including the launch of our new charitable entity.
"We look forward to sharing the full details of this exciting next step in due course, as we continue to collaborate with Her Majesty The Queen, The Prince of Wales, The Duke of Cambridge and all relevant parties. Until then, please accept our deepest thanks for your continued support."
Queen Elizabeth was reportedly left "deeply upset" by the decision and began the task of fixing her family's future at a meeting at Sandringham on Monday, which involved Harry, Prince William and Prince Charles.
After the crisis talks, the Queen said she supported Prince Harry and Meghan's "desire to create a new life as a young family" in a statement.
“Today my family had very constructive discussions on the future of my grandson and his family,” the statement read.
“My family and I are entirely supportive of Harry and Meghan’s desire to create a new life as a young family.
“Although we would have preferred them to remain full-time working Members of the Royal Family, we respect and understand their wish to live a more independent life as a family while remaining a valued part of my family."