The Queen's coffin arrives at Buckingham Palace
The King was there with Royal Family members including his wife Camilla, the Queen Consort, Princes William and Harry and their wives, Catherine and Meghan.
The hearse, accompanied by the Queen's daughter Princess Anne, travelled by road after a flight from Edinburgh.
Despite the rain, crowds gathered along the route to pay their respects.
There was applause from the thousands gathered outside as the hearse arrived at the palace. Shouts of "hip, hip hooray" were heard as the sombre mood gave way to respectful celebration of the late Queen.
Many people lining the street put down their umbrellas as a sign of respect while phone cameras lit up the crowds as people documented the historic moment.
The King and Queen Consort have since left Buckingham Palace after witnessing the arrival of the coffin.
The procession to the palace marks one of the final stages of a solemn journey, which began on Sunday at the Queen's Balmoral estate where she died aged 96 on Thursday.
In a statement released on Tuesday evening, the Princess Royal, Princess Anne, said it had been an "honour and a privilege" to accompany the Queen on her final journeys, having travelled with her mother's coffin from Balmoral to London.
"Witnessing the love and respect shown by so many on these journeys has been both humbling and uplifting," she said.
"We will all share unique memories. I offer my thanks to each and every one who share our sense of loss.
"We may have been reminded how much of her presence and contribution to our national identity we took for granted."
Princess Anne said it had been an "honour and a privilege" to accompany her mother's coffin on its journey to London
Thousands paid their respects as the Queen lay at rest in St Giles' Cathedral, in Edinburgh overnight, before her coffin was flown to RAF Northolt, in north-west London this evening.
The national anthem was played as the RAF plane, which has previously been used to evacuate people from Afghanistan and deliver weapons to Ukraine, prepared to take off.
The number of people outside the palace began to swell this afternoon - although it thinned slightly as the rain became heavier, with most of those gathered wearing raincoats and carrying umbrellas - creating a subdued atmosphere.
But there were still several thousand waiting along the barriers outside Buckingham Palace.
Among them was Londoner Alex Myers, and her dog Minnie, who decided to take a detour to the palace after hearing the Queen's coffin was on its way.
She said the atmosphere when the hearse arrived was "incredible and inspiring and very moving".
"I think the outpouring of love has been a really amazing thing," she said, as both her and Minnie headed home slightly wetter than when they arrived, hours before.
The Queen's coffin was in a bespoke new hearse designed in consultation with her, to allow the public a clear view of her casket.
The Queen's coffin will rest in the Bow Room at Buckingham Palace until it is conveyed to Westminster Hall on Wednesday
The Queen's coffin will rest in the Bow Room of the palace overnight before it is borne in procession on a gun carriage departing the palace for Westminster at exactly 14:22 BST on Wednesday.
In the past, the Queen has hosted foreign royalty and dignitaries in the room, including King Abdullah of Saudi Arabia, former US president George Bush, and Premier League footballers.
It is in the palace's west wing, was formerly known as the Bow Library and was named after the window that runs along one side.
The Queen made her last appearance at Buckingham Palace in June alongside three generations of her family
On Wednesday, the Queen's coffin will be taken via The Mall, Horse Guards Parade, Whitehall and Parliament Square to Westminster Hall.
Flights from Heathrow could face disruption to ensure silence over central London as the Queen's coffin is moved, the airport said.
The coffin will lie in state in the hall until the morning of the funeral next Monday. More than 400,000 people are expected to be able to file past it there.
From 17:00 BST on Wednesday, people will be able to pay their respects, although mourners have been warned to expect long queues.
There will be some assistance for those unable to stand for a long time, Downing Street said, with the Prime Minister's official spokesman adding that "everything possible" would be done to facilitate those who could not wait for extended periods.
Barriers for the queue were being placed as far out as Southwark Park, in south-east London, about three miles from Westminster, and about 50 people had already begun queuing to see the Queen lying in state by 20:30 BST on Tuesday.
The Queen's final public appearance at Buckingham Palace was on the east front balcony on 5 June, where she stood alongside family members during Platinum Jubilee celebrations.
Earlier, after standing vigil by his mother's coffin in Edinburgh on Monday, King Charles travelled to Northern Ireland alongside Camilla, the Queen Consort, where he met politicians and officials.
He said the Queen had "never ceased to pray for the best of times for this place and its people" and said she had felt deeply "the significance of the role she herself played in bringing together those whom history had separated".
The King pledged to "seek the welfare of all the inhabitants of Northern Ireland".
He received a message of condolence from the speaker of the assembly Alex Maskey, a member of Sinn Féin, who said the Queen recognised how a "small but significant gesture can make a huge difference in changing attitudes".
More than 26,000 mourners filed past the Queen's coffin at St Giles', where she lay at rest before being taken to Edinburgh airport and flown to RAF Northolt, in West London.
People queued for hours with the final wristbands to granting access being given out at 13:00 BST, leaving some of those turned away in tears.
-- Courtesy of BBC News