The Duke and Duchess of Sussex are attending their final official royal engagement before they step down as working members of the royal family before they step down from their roles on 31 March.
The Commonwealth Service is held every year on the second Monday of March, and is attended by heads of government and representatives from the 54 countries of the Commonwealth.
The occasion will be the last time Meghan and Prince Harry make a public appearance as working members of the royal family, before quitting their duties on 31 March.
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The couple are set to attend the Commonwealth Service at Westminster Abbey, which begins at 3pm and ends at 4pm.
Instead, the couple will be escorted to their seats after the Earl and Countess of Wessex.
The service will be televised live on BBC One from 2.15pm.
It has been reported that last-minute changes made on Sunday mean that the Duke and Duchess of Cambridge will also not join the Queen for the Commonwealth Day service procession.
According to a source, the alterations were made despite the Order of Service having already been signed off and printed.
Neither Kensington Palace and Buckingham Palace have commented on why the last-minute switch was made.
Last week, Meghan took part in her final solo engagement as a working member of the royal family.
The duchess made a surprise visit to Royal Clack Upper School in Dagenham ahead of International Women's Day.
During the visit, the school's head boy, Aker Okoye, joined Meghan on stage, where he spoke to his fellow pupils about the importance of the annual event.
The teenage schoolboy who hugged the Duchess of Sussex during her solo visit to Royal Clack Upper School wrote to Prince Harry, stating: "I hope you didn't mind me cuddling your wife."
He added: "It was a pleasure to hear her speech and to speak in front of her as well."
During last year's Commonwealth Service at Westminster, the royal family exited Westminster Abbey as part of a procession.
This year, neither the Sussexes nor the Cambridges will join the Queen for the procession during the commemorative event.
On Thursday 5 March, the Duke and Duchess of Sussex attended The Endeavour Fund Awards at Mansion House in London.
The duchess wore a simple turquoise dress with cap sleeves designed by Victoria Beckham for the event.
The occasion marked Meghan and Prince Harry's first official joint public engagement since announcing their decision to step down as senior royals.
On Saturday night, the couple attended the Mountbatten Festival of Music.
The couple were given a standing ovation as they entered London's Royal Albert Hall to take their seats, Meghan in a red gown by Safiyaa and Prince Harry dressed in his Captain General of the Royal Marines uniform.
A crowd of people lined up to go into Westminster Abbey ahead of today's service.
Spectators waited in anticipation for Meghan and Prince Harry to arrive for their final official royal engagement.
Westminster Abbey shared a tweet outlining who will be in attendance at today's Commonwealth Service.
In addition to Craig David and Alexandra Burke performing, Anthony Joshua and poet Suli Breaks will deliver reflections during the service.
Highland dancers and musicians performed outside Westminster Abbey ahead of the service for Commonwealth Day.
Geri Horner arrives at Westminster Abbey with her daughter, Bluebell Madonna Halliwell.
The theme for this year's Commonwealth Day is "Delivering a Common Future: Connecting, Innovating and Transforming".
It was chosen because "the governments and people of our diverse family of 54 Commonwealth countries connect at many levels through extensive and deep-rooted networks of friendship and goodwill", the Commonwealth website explains.
"Together, the countries and people of the Commonwealth tackle climate change and its impacts. We work to find sustainable ways of using the natural resources of our planet, its land and ocean, so that our environment is protected," it stated.
"Together, we work to build peace and harmony through social and democratic development."
Prime minister Boris Johnson and his partner, Carrie Symonds, arrive for the service.
The couple recently announced that they are engaged and expecting their first child together.
The Duke and Duchess of Sussex have arrived at Westminster Abbey for the Commonwealth Day service.
In January, the couple announced their decision to "step back" as "senior" members of the royal family.
In a statement published on their Instagram account, which is named "Sussex Royal", they said their decision came after "many months of reflection".
“It is with your encouragement, particularly over the last few years, that we feel prepared to make this adjustment," they said.
Prince Harry is wearing a blue suit, while Meghan is dressed in an emerald green Emilia Wickstead ensemble.
The duchess' outfit features an asymmetrical cape, spurring several Twitter users to compare her to a "superhero".
She accessorised with a William Chambers fascinator and beige stiletto heels by Aquazurra.
Meghan smiles at the camera as she and Prince Harry walk into Westminster Abbey.
The Duchess of Cambridge opted for red for the Commonwealth Day service, wearing a maroon coat, matching headpiece and dark burgundy heels.
This marks the second year in a row that Catherine has worn red for the service.
The Duke and Duchess of Cambridge smile as they enter Westminster Abbey with other members of the royal family.
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