Queen Elizabeth II was “bright and focused” in her last meeting with Boris Johnson days before she died, the former prime minister said.
Last Tuesday, Mr Johnson tendered his resignation at Balmoral before the Queen appointed Liz Truss as his successor.
Two days later, on Thursday afternoon, the Queen died aged 96 at the castle in Scotland.
A photo taken by a PA Media photographer Jane Barlow showed a frail Queen with bruised skin on her hand as she greeted Ms Truss.
Soon after, her family were called to her bedside at Balmoral before her death was announced hours later.
In the last of the BBC’s series of interviews with former prime ministers, Mr Johnson described the last meeting he had with the longest-reigning monarch.
He said: “The last audience I had with her – one of the reasons why it was so shocking to hear about her death was because, in that audience, she had been absolutely on it just two days before her death.
“And she was actively focused on geopolitics, on UK politics, quoting statesmen from the 50s – it was quite extraordinary.”
To the question as to whether the Queen “seem[ed] well”, he added: “She seemed very bright, very focused – look, I’m – she was clearly not well.
“And that was the thing that I found so moving when I – we – all heard about her death two days later. I just thought how incredible that her sense of duty had kept her going in the way that it had.”
He said that it was a “pretty emotional time” when he heard of her death.
Ms Bruce asked Mr Johnson if he was ever too busy to attend his weekly audience with the Queen.
“No, you must be joking,” he said before describing the meetings with the monarch as “a fantastic break from everything else” and “a wonderful moment of tranquillity”.
As per tradition, the prime minister has regular meetings with the Queen to keep her updated on government matters.
Mr Johnson was the 14th prime minister to take office during her 70-year reign, before she appointed Ms Truss as the 15th.
Now those meetings will take place between Ms Truss and the Queen’s son, King Charles III.
The Queen’s husband, Prince Philip, died in April last year and was laid to rest a week later at a funeral in the private chapel in Windsor Castle.
The congregation wore masks and was socially distanced in line with Covid rules at the time, with the Queen sitting alone.
It later emerged that lockdown-busting parties were held in Downing Street on the eve of the funeral.
“How did you two get past that?” Mr Johnson was asked.
He said: “Because of her great sense of constitutional function she never got into that sort of conversation.
“She was really absolutely focused on what she saw as the important issues. It’s a very trusting environment, the audience with Her Majesty. So that never came up.”
In July, Mr Johnson announced his resignation after three years in office as a chain of scandals – including Partygate – saw his government turn against him.