Sir Keir Starmer has praised the King for doing a "fantastic job" after he was heckled by a senator in the Australian parliament.
The prime minister said the King was an "incredible ambassador" for the UK and the 56 independent countries that make up the Commonwealth.
He was speaking after the monarch was heckled by Australian senator Lidia Thorpe during his address to the Australian parliament on Monday.
Ms Thorpe accused the King of "genocide" against her nation's indigenous people at a welcome event in Canberra, shouting: "This is not your land. You are not my King."
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Asked what he thought about politicians "heckling the King" on Monday, Sir Keir said: "Look, I think the King is doing a fantastic job, an incredible ambassador, not just for our country, but across the Commonwealth.
"I think he's doing a fantastic job, and we should remember in the context of health, that he is out there doing his public service notwithstanding the health challenges he himself has had - so I think he's doing a great job."
Ms Thorpe is the first Aboriginal woman from the state of Victoria to become a senator. When she was sworn in she refused to take an oath of allegiance to the late Queen.
After the King addressed guests at Parliament House, Ms Thorpe, who wore a possum skin coat and carried a traditional message stick, shouted: "Give us our land back. Give us what you stole from us... Our babies, our people. You destroyed our land".
Ms Thorpe was then ushered from the building's hall and shouted: "Give us a treaty - we want a treaty with this country... this is not your land, this is not your land, you are not my King, you are not our King".
She said she was prevented from handing the King a "notice of complicity in Aboriginal genocide" according to the Statute of the International Criminal Court 1998.
The Australian senator has since defended her actions, telling Sky News' Kay Burley "he can't be our King".
"We are the real sovereigns in this country," she said. "The King lives in your country, he's from your country."
She added: "We have our bones and our skulls still in his family's possession. We want that back. We want our land back.
"And we want your King to take some leadership and sit at the table and discuss a treaty with us."
Pressed on her use of the word "genocidal", Ms Thorpe claimed that "there are thousands of massacre sites in this country from invasion and someone needs to answer for that. He is the successor, then he needs to answer".
The King, who revealed he had been diagnosed with cancer earlier this year, has paused his treatment while he embarks on his first tour to Australia as the country's monarch.
Later this week he will head to Samoa for the Commonwealth heads of government meeting (Chogm), where he will be accompanied by Sir Keir.
There has been mounting pressure from leaders of Caribbean nations to pay reparations for the impact of the transatlantic slave trade.
Last year a United Nations judge claimed the UK owes about £18tn in reparations for its involvement in slavery in 14 countries.
Downing Street has said Sir Keir remains opposed to apologising for the UK's historical role in slavery and that the issue of reparations is "not on the agenda" at the summit despite calls from some of his own MPs.
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The prime minister's official spokesperson said today: "Reparations are not on the agenda for the Commonwealth Heads of government meeting.
"The government's position on this has not changed, we do not pay reparations.
"The prime minister's attending this week's summit to discuss shared challenges and opportunities faced by the Commonwealth including driving growth across our economies."
Asked again about an apology, the spokesman added: "The position on apology remains the same, we won't be offering an apology at Chogm, but we will continue to engage with partners on the issues as we work with them to tackle the pressing challenges of today and indeed for the future generations."
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