“Cracks are already beginning to show” in the UK’s funding for the Aukus agreement that could derail the ambitious nuclear submarine plan, a British parliamentary inquiry has found, highlighting a threat to Australia’s security.

UK shipbuilding has been under-funded for decades and the country’s submarine availability is “critically low”, the House of Commons defence committee’s report found.

When the nuclear submarine HMS Anson visited Australia in February, it was Britain’s only attack-class submarine at sea. It had to be rapidly recalled to the northern hemisphere – ahead of schedule – when war broke out in the Gulf, undermining confidence in UK’s capacity and commitment to Aukus.

The defence committee’s inquiry into Aukus “has revealed shortcomings and failings in the delivery of Aukus which threaten to prevent that promise becoming a reality”, the report said.

“In the UK, political leadership – essential to secure the success of a programme of Aukus’s length, cost, and complexity – has faded. We call on the prime minister to take a more visible role in promoting and driving forward Aukus to counter the political drift that could see it derailed.”

Australia is dependent upon the UK’s ability to design and build an entirely new class of nuclear submarine, the SSN-Aukus. Any delay or failure on the UK side could leave Australia exposed without any sovereign long-term submarine capability.

While Australia will buy between three and five Virginia-class submarines from the US to cover the “capability gap” between the retirement of its existing Collins-class diesel-electric submarines and the arrival of the SSN-Aukus, the US capacity to deliver these is also in serious jeopardy.

Australia has promised the UK A$4.6bn to uplift its submarine-building capacity and has sent nearly half a billion dollars to its Ministry of Defence.

The UK has more pressing priorities – it must first build its Dreadnought class of nuclear-armed submarines – and it is structurally hampered by the fact that it has only one shipyard for building submarines, at Barrow-in-Furness. £200m has been committed to upgrade Barrow but the committee found that the timeline for improvements had “already slipped”.

“Efforts to regenerate Barrow to attract and maintain the workforce required to deliver SSN-Aukus must be properly funded,” the report said.

The report said Aukus would fail if it was seen within the UK’s defence bureaucracy as just another plan “competing for scarce resources”.

“Only strong and visible political leadership from the very top of government can counter a drift into bureaucratic obscurity and ensure that Aukus receives the funding and priority that the nation’s defence and security demands.”

The committee chair, Labour MP Tan Dhesi, said “cracks are already beginning to show” in Aukus funding.

“This cannot be allowed to happen again. Even seemingly minor shortfalls and delays snowball over time, with potentially severe consequences.”

The committee said it was also disquieted by government secrecy over the reality of Aukus progress.

In 2024 the UK’s former national security adviser, Sir Stephen Lovegrove, was appointed as the government’s Aukus adviser and commissioned to review it.

“It is deeply disappointing that more than a year after Sir Stephen Lovegrove completed his review of Aukus, the government’s commitment to issue a public version of his findings has not been fulfilled,” the report said.

“This reflects poorly on the government and is damaging to stakeholder and public confidence.”

When the Astute-class submarine HMS Anson docked in Perth in February, it was billed as a vital signal in maintaining the credibility of Britain’s commitment to Aukus. But the vessel was rapidly withdrawn and deployed to the Middle East when war broke out in Iran.

“It is clear that fulfilling this commitment has stretched the Astute fleet to – or even beyond – its limits,” the report found.

The committee heard evidence that Britain’s submarine fleet was “the smallest the UK has had in living memory” and had been stretched “to the limits of its capacity” in sending the Anson to Australia.

Dr Sidharth Kaushal, a senior research fellow at the Royal United Services Institute, said trying to fulfil Aukus meant some functions and training for the UK’s fleet would have to be abandoned.

“The risk of stretching our [attack-class] fleet is not just about the availability of hulls but, frankly, that we operate it to death,” he said.