Britain has signed contracts worth 4 billion pounds ($4.8 billion) to finance a new phase of the SSN-AUKUS next-generation attack submarine project, according to government officials.

The deals, which involve British companies BAE Systems, Rolls-Royce and Babcock, are part of the AUKUS military alliance between the United States, Australia and the United Kingdom to counter China in the Asia-Pacific region.

"(The) submarines will empower the Royal Navy to maintain our strategic advantage under the sea," UK Defence Secretary Grant Shapps said at the Conservative Party's annual conference in Manchester on Sunday.

AUKUS leaders unveiled the joint submarine project last March in a deal that will see Australia replace its diesel-powered submarines with nuclear-powered ones that have far greater stealth and range.

The plan involves Australia purchasing US nuclear-powered submarines, and then building a new model with US and British technology.

British Prime Minister Rishi Sunak praised it as "the most important multilateral defence agreement for generations".

In a separate statement, BAE Systems said that the future SSN-AUKUS "will be the largest, most powerful and most advanced submarine" supplied to the Royal Navy.

The British military equipment maker said the investment will "cover development work until 2028", and the "recruitment of more than 5,000 people" at its Barrow-in-Furness site in northern England.

The first examples of this submarine are due to be delivered in the late 2030s.

Nuclear-powered submarines are difficult to detect, can travel great distances for long periods, and can carry sophisticated cruise missiles.