Former Neighbours actor Nicola Charles, whose claims have previously been debunked by AAP FactCheck, claims Sir Keir is not prime minister in a post on X, formerly Twitter.

The ex-soap star shares screenshots of the government’s response to a Freedom of Information (FoI) request, which had asked for evidence of Sir Keir’s “oath of office”.

“BREAKING UK NEWS: Kier [sic] Starmer did not take his Oath of Office. He is not Prime Minister of the UK. He is another Belligerent Occupier,” the caption reads.

Ms Charles’s post was shared on Facebook with the caption: “Lynch him. People today are literally zombies.”

Another Facebook post reads: “They’re pushing policies that DONT apply to you at all. You have NO contract with these parasites, so why OBAY[sic]!”

Robert Hazell, a University College London (UCL) government and constitution professor, told AAP FactCheck party leaders don’t need to be sworn in as prime minister in the UK – they automatically become head of the government on accepting the monarch’s invitation.

“Starmer became prime minister when King Charles invited him to form a new government, and he accepted,” Professor Hazell said.

UK Parliament outlines exactly how a prime minister is appointed on its website, and international media widely reported that King Charles invited Sir Keir to form a new government following Labour’s victory at July’s general election.

Prof Hazell said all prime ministers must take two oaths: one to become a member of parliament (MP), and one to become “First Lord of the Treasury”.

After becoming prime minister, Sir Keir was sworn in as First Lord of the Treasury on July 6 and as a member of the new parliament on July 9.

On both occasions, the prime minister made an affirmation, a non-religious oath used for legal declarations.

In the FoI request shared by Ms Charles, the Government Legal Department pointed out that Sir Keir’s swearing in as First Lord of the Treasury had been recorded by the Privy Council (page 11).

A YouTube video shared by the Independent newspaper shows Sir Keir being sworn in as an MP (one hour 39 minutes 53 seconds).

Meg Russell, also an expert on the UK’s parliament, said the claim’s focus on an oath was a red herring.

“The UK famously has no codified constitution – no document by that title, which has the status of higher law,” Professor Russell told AAP FactCheck, adding that the UK’s system relies on conventions and it is “more or less inconceivable” that the position of the prime minister could be challenged in the courts.

“The nature of our constitution means there’s no legal route to challenge the decision of the monarch,” she said.

“He’s the prime minister by virtue of being appointed by the monarch.

“If the House of Commons voted no confidence in him, by convention he’d be removed and the monarch would appoint somebody else … or else call a general election.”