The UK, US and Australia will work together “keeping our oceans free” with a new generation of nuclear-powered attack submarines, Rishi Sunak has said.

Mr Sunak met US President Joe Biden and Australian Prime Minister Anthony Albanese in San Diego to announce the next stage of the Aukus partnership plan.

The plan will deliver nuclear-powered submarines to Australia as it seeks to counter Chinese activities in the Pacific Ocean.

First announced in 2021, the Aukus partnership was Australia’s reaction to China’s actions. Since then, the UK has published its integrated review of foreign and security policy, which highlights China’s “more aggressive stance”.

“For the first time ever, it will mean three fleets of submarines working together across both the Atlantic and Pacific, keeping our oceans free, open, and prosperous for decades to come,” Mr Sunak said.

He added that the collaboration would deliver “one of the most advanced” submarines “the world has ever known” and will replace the UK’s seven Astute-class submarines.

Earlier on Monday, at a security and foreign policy review, Mr Sunak announced £5bn extra for defence, which will partly help develop the next phase of the Aukus programme.

The UK will also contribute expertise to build the new SSN-Aukus boats, creating jobs in British shipyards in Barrow-in-Furness and Derby.

They will be in operation for the Royal Navy by the late 2030s.

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There is no confirmed number of how many will be ordered, but the size of the hunter-killer fleet could double, Royal Navy insiders said.

As part of the deal, the US intends to sell Australia three US Virginia class nuclear-powered submarines.

President Biden said the US could ask for “no better partners” than the UK and Australia when it came to ensuring the security of the Pacific.

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Why is Rishi Sunak in San Diego?

Mr Albanese said the programme will result in a $6bn Australian dollar (£3.3bn) investment in its industrial capability over the next four years.

Australia’s boats will be built in the south of the country, using some components manufactured in the UK, and will be in service in the early 2040s.

The three nations insisted that the deal did not increase the risk of nuclear proliferation. The nuclear reactors will be sealed shut with the vessels carrying conventional weapons.

Biden accepts invite to visit Northern Ireland

During the same visit, Mr Biden said he intends to visit Northern Ireland after being invited to mark the 25th anniversary of the Good Friday Agreement.

As Mr Sunak met the president in Point Loma naval base, San Diego, Mr Biden said: “It’s my intention to go to Northern Ireland and the Republic.”

When inviting the president, Mr Sunak said “we’d love to have you over” to which Mr Biden replied: “Twenty-five years? It seems like yesterday.”