The King will not attend next month’s COP29 climate conference, Sky News understands.
The monarch is well known for championing environmental causes and gave a speech at the opening of last year’s event in Dubai.
The King, 75, is continuing his treatment for an undisclosed form of cancer.
However, he will put it on hold for the 11 days he is away from home on a royal tour of Australia and Samoa, which begins next week.
A plan to also visit New Zealand had to be cancelled on doctor’s advice and adjustments were made to the rest of the schedule to allow more time for rest.
When the trip was announced, a Buckingham Palace spokesman said: “We’ve had to make some difficult decisions about the programme with the Australian government, about where their majesties can get to.”
The King’s only overseas visit since he announced his cancer diagnosis in February was to Normandy in June for the D-Day commemorations.
COP29 begins in Baku, Azerbaijan, on 11 November, just over a fortnight after the royals’ long flight back to the UK.
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At last year’s COP, the King urged world leaders to take “real action” on the climate crisis to steer people and nations “away from practices that make our world more dangerous”.
He missed the 2022 summit, the first of his reign, after a “unanimous agreement” was reached he should not attend the gathering in Egypt following advice from the government.