The Duke of Cambridge and the Prime Minister will join families of the victims of the Manchester Arena atrocity one year on at a remembrance service in the city.
They will attend the ceremony at Manchester Cathedral along with some of the hundreds injured in the May 22 terror attack, first responders to the scene, civic leaders and other national figures.
The invitation-only service, held between 2pm and 3pm, will incorporate a national silence at 2.30pm, which will also be marked at UK government buildings.
Members of the public will be able to watch proceedings on a big screen in nearby Cathedral Gardens, while the service will also be screened at York Minster, Liverpool Metropolitan Cathedral and Glasgow Cathedral.
Prince William will privately meet some of the bereaved families following the multi-faith service.
Writing in the Manchester Evening News, Theresa May said: "The targeting of the young and innocent as they enjoyed a carefree night out in the Manchester Arena on May 22, 2017 was an act of sickening cowardice.
"It was designed to strike at the heart of our values and our way of life in one of our most vibrant cities, with the aim of breaking our resolve and dividing us. It failed.
"For such appalling acts of wickedness will do nothing but strengthen our resolve to defeat such twisted ideologies and beliefs.
"The resilience and determination shown by this city in the 12 months since is testament to that."
Later on Tuesday, more than 3,000 singers from local choirs will join forces and share the spirit of solidarity at the Manchester Together - With One Voice event in the city's Albert Square from 7.30pm-9pm.
Among those performing are the Manchester Survivors Choir, a group made up people who were at the Arena on the night of the fateful Ariane Grande concert, and Parrs Wood High School's Harmony Group, whose post-attack tribute went viral last year.
A mass 30-minute communal singalong finale promises to be the highlight of the concert, with songs including Ariana Grande's One Last Time, One Day Like This by Elbow, Don't Look Back In Anger by Oasis and Never Forget by Take That.
At 10.31pm, bells will ring out from the city's Town Hall, St Ann's Church and St Mary's RC Church to mark the moment when the attack took place 12 months ago.
Salman Abedi, 22, detonated his bomb device at the end of the concert with 353 people, including 175 children, around him in the foyer of the venue.
A total of 22 people were killed and more than 800 others were either physically or psychologically injured.