Britain will work with the US and Australia in developing hypersonic weapons, after Russia used the deadly high-speed missiles in airstrikes last month during the war in Ukraine. The military agreement – endorsed by Joe Biden, Boris Johnson and the Australian prime minister, Scott Morrison – is a new element in the Aukus pact, originally announced last autumn to provide nuclear-powered submarines to Canberra. A statement from all three leaders announced a further expansion of the agreement, described as “new trilateral cooperation on hypersonics and counter-hypersonic” weapons, part of a growing militarisation after the Russian invasion. China has tested its own version of the weapons, and the Aukus leaders are keen to be seen to be presenting a united front against Beijing and Moscow. “Our ability to determine their use and to counter their use will be important part of maintaining stability right across the world,” a British official said. The US quietly tested its own hypersonic missile last month, although it kept details secret for a fortnight for fear of antagonising Russia, and it is already collaborating with Australia. The agreement means Britain is now engaged for the first time. Stephen Lovegrove, the UK’s national security adviser, said the latest announcement demonstrated the development of Aukus. “In light of Russia’s invasion of Ukraine, it’s more important now than ever that allies work together to defend democracy, international law and freedom around the world,” he added. Hypersonic missiles – which can carry nuclear or conventional warheads – travel at at least five times the speed of sound, faster than conventional cruise missiles. They are designed to be manoeuvrable, allowing them, in theory, to evade conventional missile defences. However, it is unclear if they have any genuine military utility, and critics say their development is contributing to a deepening international rearmament in response to the Ukraine crisis. Biden said last month that Russia had used its Kinzhal hypersonic missile against Ukrainian targets, describing it as “a consequential weapon” that was “almost impossible to stop”. Russia said it had used them twice, against targets near Mykolaiv and elsewhere in the west of Ukraine. Vladimir Putin has previously praised the Kinzhal and weapons like it, saying four years ago that “its speed makes it invulnerable” and that it was “quite understandable why the leading armies of the world seek to possess such an ideal weapon”. But others have voiced scepticism. The US defence secretary, Lloyd Austin, said he would “not see it as a gamechanger”, and other experts see them simply as modified versions of Russian Iskander ballistic missiles, which are launched from a plane against a target. China has been testing hypersonic weapons using a different technology, known as the hypersonic glide vehicle, in which the missiles are initially launched into space and attached to dedicated aerodynamic craft designed to bring them down to their targets on Earth. The UK, US and Australia intend to work on researching both types of hypersonic technology – missiles and glide vehicles – and examine ways of countering the threat they may pose. “We need to understand how the overall targeting system of our adversaries might work,” a British official said. Pentagon officials have assessed that hypersonic weapons will add $21.5bn (£16.4bn) to the navy’s budget and $7bn to the army’s in coming years, although the estimates are tentative as the technology emerges. British sources, however, said the UK had not committed to buying any hypersonic weapons, and is only engaged in a research and development phase before deciding how to proceed. Kate Hudson, the general secretary of the pressure group CND, said she believed the extension of the Aukus pact would “further escalate global tensions at a time when the threat of nuclear war is at its highest in decades”, and it risked accelerating an arms race with China in the Asia-Pacific. “Not to mention the fact that military budgets are already escalating – what will the opportunity cost be for embarking on a whole new class of weaponry?” Hudson added. The joint progress report will be seen as politically helpful to Morrison, whose Coalition government in Canberra is trailing Labor in the polls as it prepares for a federal election expected on 14 or 21 May. Morrison, who is expected to formally trigger the election campaign within days, is seeking to make national security one of his key themes, arguing that the world is facing “uncertain times” and it is not a moment for “weakness”. • This article was amended on 7 April 2022 because an earlier version referred to “nuclear-capable” hypersonic weapons. To clarify: such weapons are in theory nuclear-capable but those to be developed will not be.