The partnership will enable Australia to build nuclear-powered submarines for the first time.

The pact, to be known as Aukus, will also cover artificial intelligence, quantum technologies and cyber.

The three countries are worried about China's growing power and military presence in the Indo-Pacific.

As a result of the pact, Australia has scrapped a deal to build French-designed submarines.

France won a A$50bn (€31bn; £27bn) contract to build 12 submarines for the Australian Navy in 2016. The deal was Australia's largest-ever defence contract.

However, the project was hit with delays largely because of Canberra's requirement that many components be sourced locally.

On Wednesday, US President Joe Biden, UK Prime Minister Boris Johnson and his Australian counterpart Scott Morrison issued a joint statement on the launch of the new security partnership, labelled Aukus.

"As the first initiative under Aukus... we commit to a shared ambition to support Australia in acquiring nuclear-powered submarines for the Royal Australian Navy," the statement said.

"This capability will promote stability in the Indo-Pacific and will be deployed in support of our shared values and interests," it said.

The leaders said the aim was to "bring an Australian capability into service at the earliest achievable date", adding: "Australia remains committed to fulfilling all of its obligations as a non-nuclear weapons state."

It went on to say that the defence pact would also focus on cyber capabilities, artificial intelligence and "additional undersea capabilities".

The UK Government says this is a very significant defence agreement - a point reinforced by the fact that the leaders of Britain, the United States and Australia have appeared together by video conference to announce this partnership. It also underlines the growing importance of the Indo-Pacific region to both the US and the UK.

The second is China. Though British officials insist the new defence agreement is not a response to any one country, the UK Government does say it is about ensuring prosperity, security and stability in the region and supporting a peaceful "rules-based order". And it is no secret that Britain, the US and Australia share concerns about China's military build up in the Indo-Pacific.

Mr Johnson said the three nations were natural allies and that the alliance would "bring us closer than ever".

"This partnership will become increasingly vital for defending our interests, and... protecting our people back at home," he said.

In recent weeks, the UK's HMS Queen Elizabeth aircraft carrier has been deployed to the Indo-Pacific region alongside personnel and equipment from the US.

The joint statement said the Indo-Pacific was a region with potential flashpoints, including unresolved territorial disputes, threats from terrorism and a problem with organised crime. "It is on the frontline of new security challenges, including in cyberspace," it said.