Boris Johnson has started interviewing candidates to become his new military chief, with an announcement expected early next month, Sky News understands.

The prime minister is thought to have begun the selection process for this pivotal role on Thursday after his return from a trip to the US.

More interviews are set to take place next week, with sources saying the competition to replace General Sir Nick Carter, the incumbent chief of the defence staff, is still “wide open”.

The new head of the armed forces will have to deal with the challenges of a rising China and threats from Russia at the same time as a likely increased terrorist threat following the US-led withdrawal from Afghanistan.

It is thought that five candidates are in the running – the heads of the Royal Navy, army and Royal Air Force, as well as the commander of Strategic Command and – unusually – a slightly lower-ranked officer who oversaw the UK’s Afghan evacuation operation.

The appointment of Britain’s new military chief is dubbed “the Game of Thrones” in defence circles and follows months of increasingly heated speculation over who is in the running.

The prime minister makes the final selection, but he was given a list of prospective candidates from Ben Wallace, the defence secretary, who has already conducted interviews.

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A favourite for the job is Admiral Sir Tony Radakin, the first sea lord – and not just because the last time the navy produced a military chief was then Admiral Sir Michael Boyce in 2001.

Admiral Radakin – who is often active on social media – played a part in helping to forge a landmark defence partnership between Australia, the UK, and the US (AUKUS), which will begin with a long-term collaboration on nuclear-powered submarines.

The navy is also enjoying once again operating aircraft carriers after a decade without, while the prime minister vowed to “restore Britain’s position as the foremost naval power in Europe” when he unveiled a plan to boost defence spending last year.

In addition, Mr Johnson apparently appreciated an announcement by the navy on Thursday to make James Bond actor Daniel Craig an honorary commander – like the on-screen spy.

“The PM absolutely adored that,” one defence source said.

Another candidate to watch is General Sir Mark Carleton-Smith, chief of the general staff.

He has – by choice – a lower media profile than some of the other candidates, and some in defence circles don’t rate his chances.

But others say he is the one to look out for, noting his experience as a former director of special forces and the fact that he has a good, personal relationship with Mr Johnson – the two men, both former Etonians, play cricket together.

“He does like Carleton-Smith,” the defence source said.

The army chief “delivered for him on COVID – produced soldiers and military planners”.

The source was referring to how the armed forces, not just the army, stepped up to help support the Department of Health and Social Care as well as other parts of the government during the toughest days of the pandemic.

Counting against General Carleton-Smith, though, is the army’s beleaguered, multi-billion-pound procurement of a new fleet of armoured vehicles, Ajax, which is beset with delays and problems.

The procurement did not happen on his watch, but it is a challenge he has been left with.

A third candidate regarded as being in with a chance is General Sir Patrick Sanders, who runs Strategic Command, which is in charge of special forces and cyber.

It plays a leading role in contesting Britain’s opponents in the “grey zone” below the threshold of war – an increasingly prominent area, giving General Sanders an advantage.

Air Chief Marshal Sir Mike Wigston, the head of the RAF, is also in the process, though defence sources said that he is seen as unlikely to be selected.

Finally, and most unusually, a three-star officer is among the runners and riders.

Mr Johnson is thought to have been keen to open the selection process to the next rung down from the full, four-star generals, admirals and air chief marshals who apply.

It means Vice Admiral Sir Ben Key, commander of joint operations, is also being interviewed.

He ran Operation Pitting, the UK’s evacuation operation from Afghanistan.

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“He is a lovely bloke,” the defence source said. “He is really good, scored highly in Op Pitting.”

General Carter is due to retire from the military in November.

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