The Royal Family is to lead the UK's commemorations of the 75th anniversary of VJ Day - the day World War Two ended with Japan's surrender.
The Prince of Wales will attend a memorial while the Duke of Cambridge will appear in a special TV programme.
The Duke of Edinburgh will also feature in commemorations, appearing on screens across the country in a photo montage with other veterans.
The PM paid tribute to veterans for restoring "peace and prosperity".
VJ Day - or Victory over Japan Day - on 15 August 1945 ended one of the worst episodes in British military history, during which tens of thousands of servicemen were forced to endure the brutalities of prisoner of war camps.
It is estimated that there were 71,000 British and Commonwealth casualties of the war against Japan, including more than 12,000 prisoners of war who died in Japanese captivity. More than 2.5 million Japanese military personnel and civilians are believed to have died over the course of the conflict.
The fighting in Europe had ended in May 1945, but many Allied servicemen were still fighting against Japan in east Asia.
Japan rejected an ultimatum for peace, and the US believed that dropping a nuclear bomb would force them to surrender. The US dropped two atomic bombs on Japan, killing an estimated 214,000 people, and two weeks later Japan surrendered.
To mark the 75th anniversary, Japan's Prime Minister Shinzo Abe sent an offering to a controversial war shrine in Tokyo, but did not attend in person.
However, two of his ministers did visit the Yasukuni Shrine - in which 14 leaders who were later convicted by the Allies as war criminals are commemorated.
What's taking place today?
The commemorations will begin at sunrise, with a piper playing Battle's Over at the Imperial War Museum's HMS Belfast in London.
Military pipers will also be playing at dawn in India, Australia, New Zealand and Nepal. In Japan, national memorial services will be held in Tokyo at 04:00 BST.
At 11:00 BST, Prince Charles and the Duchess of Cornwall will attend a national service of remembrance and lead a two-minute silence from the National Memorial Arboretum in Staffordshire. The service will be broadcast on the BBC.
Mr Johnson will read the Exhortation - the part of the war poem beginning "they shall grow not old" - before the nation falls silent, and afterwards the Battle of Britain Memorial Flight will take part in a flypast.
The Red Arrows will perform a flypast over the cities of Edinburgh (11:30), Belfast (14:00), Cardiff (14:45) and London (17:30) - the first time such a flypast will take place since the London Olympics.
Image copyright Reuters Image caption The flypast will be one of the few physical events marking VJ Day this year due to coronavirusA photo montage featuring Prince Philip and other veterans will be shown on a number of large screens across the UK.
On Saturday evening, the BBC will air a pre-recorded programmed from 20:30 called VJ Day 75: The Nation's Tribute, which will tell the story of those who served in the Far East and include a message from Prince William.
Boris Johnson has joined other world leaders including US President Donald Trump in recording a video message to thank veterans.
In the video, each leader says in turn: "To all who served, we thank you."
Image copyright Getty Images Image caption At the 70th anniversary of VJ Day there was a parade in LondonMr Johnson added: "On this 75th anniversary of the end of the Second World War, we pay tribute to the heroes deployed thousands of miles away in the mountains, islands and rainforests of Asia.
"Unable to celebrate the victory in Europe, and among the last to return home, today we recognise the bravery and ingenuity of those who, in the face of adversity, restored peace and prosperity to the world.
"Their immeasurable sacrifice changed the course of history and, at today's commemorations, we take the opportunity to say what should be said every day - thank you."
'Courage in adversity'
In a letter specifically addressed to Far East veterans, Mr Johnson said: "You were the last to come home but your achievements are written in the lights of the glittering capitals of the dynamic region we see today."
"All of us who were born after you have benefited from your courage in adversity. On this anniversary, and every day hereafter, you will be remembered," he added.
Labour leader Sir Keir Starmer also recorded a message, paying tribute "to the wartime generation, who through the horrors of conflict showed us the spirit and determination that we need to always remember and always be grateful for".
"It's important that as we face the challenges of today, we take inspiration from that generation," he said.
Prince Philip, 99, was a young Royal Navy officer aboard a warship in Tokyo Bay when Japan surrendered.
The photo montage will be a rare appearance for the duke, who has only been seen a handful of times in public since retiring in 2017 - most recently for a military event at Windsor Castle.
Meanwhile Capt Sir Tom Moore, who served in the Burma campaign has previously encouraged the public to join in the commemorations, describing VJ Day as "the most special day".
"It was VJ Day when the pain of war could finally start to fall away as peace was declared on all fronts," said Sir Tom - who raised millions of pounds for NHS charities by walking laps of his garden during lockdown.
"I respectfully ask Britain to stop whatever it is doing and take some time to remember.
"We must all take the time to stop, think and be thankful that were it not for the ultimate sacrifices made all those years ago by such a brave band of men and women, we would not be enjoying the freedoms we have today, even in these current difficult times."
BBC One will broadcast live from the National Memorial Arboretum from 9:30 BST.
VJ Day 75: The Nation's Tribute will be aired from 20:30 BST .