BAE Systems said the Ministry of Defence had awarded it the funding to cover development work to 2028, allowing it to start detailed design work on the submarines.

British defence firm Babcock Marine said it has also agreed a five-year contract with the government to provide input into the detailed design for the new submarines.

In March, the UK, US and Australia announced details of the AUKUS plan to create a new fleet of nuclear-powered attack submarines to patrol the Pacific Ocean from the early 2030s.

The partnership has been set up to help counter China's ambitions in the Indo-Pacific region.

China criticised the pact, saying it had "disregarded" the concerns of the international community and the three countries involved had "gone further down a dangerous road".

Defence Secretary Grant Shapps said the submarines, which will be nuclear-powered and conventionally armed, would be the "most powerful attack submarines ever operated by the Royal Navy".

He said: "This multi-billion-pound investment in the AUKUS submarine programme will help deliver the long-term hunter-killer submarine capabilities the UK needs."

Mr Shapps added the submarines will also "empower the Royal Navy to maintain our strategic advantage under the sea".

The contract will secure funding for infrastructure work at the BAE Systems site in Barrow-in-Furness, Cumbria, and the company said it would help fund 5,000 jobs.

BAE Systems, which is Britain's biggest defence firm and employs more than 93,000 in around 40 countries, said the submarines will gradually replace the Astute class, which it builds.

Manufacturing of the submarines would start towards the end of the decade, with the first SSN-AUKUS vessel due to be delivered in the late 2030s.

"This funding reinforces the government's support to our UK submarine enterprise and allows us to mature the design, and invest in critical skills and infrastructure to support our long-term national security," said Charles Woodburn, who is the chief executive at BAE Systems.

Read more:
What is the AUKUS submarines pact?

The UK government said Australia will work over the next decade to build up its "submarine industrial base".

The first Australian submarines will become available in the early 2040s, the government added.

Rolls-Royce will be supplying the nuclear reactors for all UK and Australian submarines.