The Duke and Duchess of Sussex are due to make their final public appearance as working members of the Royal Family.
Prince Harry and Meghan will join the Queen and other royals at the Commonwealth Day Service at Westminster Abbey on Monday afternoon.
The couple have been carrying out a series of public appearances in the UK before stepping back as working royals.
From 31 March, they will stop using their HRH titles and receiving public money.
The duke and duchess will gather with the Queen – who is head of the Commonwealth – the Prince of Wales, the Duchess of Cornwall and the Duke and Duchess of Cambridge in the central London cathedral.
Last-minute changes mean the Cambridges and the Sussexes will go straight to their seats – rather than waiting for the Queen and taking part in the procession as they did in 2019.
Kensington Palace and Buckingham Palace have not said why the late amendment was made.
It will be the first time Sussexes have appeared with other members of the Royal Family since announcing their intention to “step back” as senior royals in January.
After the service, they are expected to return to their current base in Canada, where their son, Archie, has remained during the UK trip.
It is where the couple are to begin their new life of personal independence, pursuing private commercial deals and charity projects.
Prince Harry and Meghan will retain use of Frogmore Cottage, in Windsor, and aides have said they will be in the UK regularly.
They will still attend some royal events but these will not be classed as official duties.
The new arrangements will be reviewed next year.
The duke and duchess have conducted a farewell tour of the UK with several appearances including the Endeavour Fund Awards and a military musical festival at the Royal Albert Hall.
Meghan made a surprise visit to a school in Dagenham, east London, to celebrate International Women’s Day.
Meanwhile, the duke joined Formula 1 driver Lewis Hamilton to visit a new motor racing museum at the Silverstone Circuit.
The couple have made supporting the Commonwealth a priority for their royal duties and overseas visits.
In stepping down as working royals, the duke will relinquish his role as Commonwealth Youth Ambassador.
But Harry will remain president of the Queen’s Commonwealth Trust and Meghan will still be the Trust’s vice-president.
In her Commonwealth Day message, the Queen has praised the diversity of the family of nations whose blend of traditions “serves to make us stronger”.
Dr Linda Yueh, chair of the Royal Commonwealth Society, said “contributing from far away” will be a key theme of the service.
“That’s probably the hope, that even as the Duke and Duchess of Sussex have stepped down from formal duties they’ll still be contributing in a less formal capacity to the Commonwealth in the years to come,” she added.
Heavyweight boxing champion and Olympic gold medallist Anthony Joshua will deliver a reflection at the service, while singers Alexandra Burke and Craig David will perform.
The Duke of York will be absent from this year’s service, having resigned from royal duties following criticism of his BBC Newsnight interview over his friendship with convicted sex offender Jeffrey Epstein.
The Commonwealth Day service will be broadcast live on BBC One from 14:15 GMT and across the BBC World Service.