British pop superstar Ed Sheeran was on Sunday set to bring the curtain down on four days of momentous nationwide celebrations to honour Queen Elizabeth II's historic Platinum Jubilee. 

The multi-award-winning singer-songwriter will perform at the finale of a day-long pageant lauding the 96-year-old monarch's record seven decades on the throne, as a long weekend of festivities across the UK concludes.

Sheeran is one of numerous "national treasures" poised to perform a "special tribute" to the queen against the backdrop of Buckingham Palace to mark the milestone never previously reached by a British sovereign.

Meanwhile, millions of people are expected to attend "Big Jubilee Lunch" picnics, including an attempted world record for the longest street party.

It remains unclear if the queen will make any in-person appearances at the pageant, after being forced to skip several Platinum Jubilee celebration appearances due to mobility issues.

The four-day extravaganza began Thursday with the pomp and pageantry of the Trooping the Colour military parade to mark her official birthday, followed by beacon-lighting ceremonies across the country.

She made two public appearances to huge crowds on the Buckingham Palace balcony that day, and then launched the beacon-lighting at Windsor Castle.

Friday's focus was a traditional Church of England service of thanksgiving led by senior royals -- and returning Prince Harry and his wife Meghan -- in the hallowed surroundings of St Paul's Cathedral in London.

Then on Saturday, the tone turned more celebratory as Motown legend Diana Ross and Italian opera legend Andrea Bocelli led a star-studded "Platinum Party" outside Buckingham Palace.

- Spectacle -
Sunday's "Platinum Jubilee Pageant" will kick off with a military spectacle celebrating Britain's armed forces along with personnel from many of the other 53 Commonwealth countries.

The Mounted Band of the Household Cavalry -- the largest regular military band in the UK -- will lead the Gold State Coach along a crowd-thronged route to Buckingham Palace.

A cast of 10,000 will then stage a street performance showcasing popular culture over the seven decades of the queen's reign featuring music, dance, fashion, youth culture and classic cars.

Performers from street theatre, carnival and other genres will also join in to celebrate her extraordinary life.

Highlights will include an aerial artist suspended under a vast helium balloon, known as a heliosphere, bearing the sovereign's image.

The carnival will include a giant oak tree flanked with maypole dancers, a huge moving wedding cake sounding out Bollywood hits, a towering dragon and three-storey-high beasts.

The spectacle will culminate in the singing of Britain's national anthem, "God Save the Queen", and Sheeran's much-anticipated performance.

Earlier on Sunday, up to 10 million people are expected to take part in the Big Jubilee Lunch picnics nationwide.

More than 70,000 had registered to host such picnics in villages, town and cities, with families, neighbours and entire communities set to come together to share food and drink.

More than 600 lunches have also been planned throughout the Commonwealth and beyond, from Canada to Brazil, New Zealand to Japan and South Africa to Switzerland.

A flagship feast with specially invited guests will take place at The Oval cricket ground in London. 

- 'Full circle' -
Sheeran, 31, will wrap up the Platinum Jubilee celebrations by singing his 2017 hit "Perfect".

Ahead of his appearance, the "Shape of You" singer-songwriter revealed that the 2002 "Party at the Palace" to mark the queen's Golden Jubilee actually inspired his phenomenally successful musical career.

Watching on television, he saw Eric Clapton play his classic song "Layla" and decided "that's what I wanna do", he wrote on Instagram.

Sheeran went on to perform at the queen's Diamond Jubilee concert 10 years ago. 

"Life is weird how it keeps coming full circle in lovely ways," he added.

His headline performance will follow Saturday night's "Platinum Party", which featured an array of stars on stage outside Buckingham Palace, with Prince Charles and his son Prince William paying personal tribute to the queen's decades of service.

"You pledged to serve your whole life -- you continue to deliver," Charles said in his poignant message to "Mummy", which he capped by calling for "three cheers to Her Majesty".

The nearly three-hour concert, watched on TV by the monarch from Windsor, came after two packed days of celebrations on Thursday and Friday, which were designated public holidays.

Longer pub opening hours, the various street parties and other events celebrating the queen's central place in the life of most living Britons have been credited with temporarily lifting the gloom of a worsening cost-of-living crisis.