Queen Elizabeth II is in good spirits, British Prime Minister Boris Johnson stressed on Saturday after doctors advised the 95-year-old monarch to rest for at least a fortnight.

"I spoke to Her Majesty and she's on very good form," Johnson told UK broadcaster Channel 4 in Rome, where he is attending a weekend G20 summit.

"She's just got to follow the advice of her doctors and get some rest, and I think that's the important thing. I think the whole country wishes her well."

The Queen, who recently spent a night in hospital for "preliminary examinations", is following medical advice to recuperate for at least the next two weeks, Buckingham Palace said on Friday.

She has cancelled her appearance at the COP26 UN climate conference in Glasgow, a rare move for the workaholic monarch and all the more telling given her personal commitment to environmental issues.

Johnson, who will preside over a COP26 summit of more than 120 leaders on Monday and Tuesday, warned of the existential stakes at play as he conducted a round of interviews in Rome's ancient Colosseum.

The Roman Empire ultimately collapsed and the Dark Ages followed in Europe. "It's important to remember things can get dramatically worse," he told Channel 4.