Britain is to start working with the US and Australia to develop hypersonic weapons to counter the threat posed from Russia and China.
Russia claims to have used its Kinzhal hypersonic missiles in strikes on Ukraine last month, which appears to be the first time they have been used in war.
And China has also been ramping up its development of hypersonic weapons that are reportedly capable of travelling up to 25 times the speed of sound.
Now the UK has agreed to cooperate with the United States and Australia on hypersonic weapons and electronic warfare capabilities following a call between leaders of the new defence pact.
The AUKUS alliance, launched last September, led to Australia cancelling a contract for a French submarine in favour of a nuclear submarine programme supported by the United States and Britain - sparking outrage from the French government.
In a joint statement, AUKUS leaders Johnson, US President Joe Biden and Australian Prime Minister Scott Morrison said they were pleased with the progress of the programme for conventionally armed, nuclear-powered submarines for Australia, and that the allies would co-operate in other areas too.
"We also committed today to commence new trilateral cooperation on hypersonics and counter-hypersonics, and electronic warfare capabilities," the statement said.
The US and Australia already have a hypersonic weapon programme called SCIFiRE, an acronym for Southern Cross Integrated Flight Research Experiment.
British officials said that though Britain would not be joining that agreement, the countries would work together on research and development in the area to expand options.
Biden's administration is investing in the research and development of hypersonic missiles as Russia's February invasion of Ukraine has intensified concerns about European security.
"In light of Russia's unprovoked, unjustified, and unlawful invasion of Ukraine, we reiterated our unwavering commitment to an international system that respects human rights, the rule of law, and the peaceful resolution of disputes free from coercion," the leaders said, adding they also reaffirmed their commitment to a "free and open Indo-Pacific".
Russia says it launched a "special military operation" in Ukraine on February 24 to demilitarize its neighbour.
Asked about the cooperation deal between Britain, the United States and Australia on hypersonic weapons, China's UN Ambassador Zhang Jun warned against measures that could fuel a crisis like the Ukraine conflict in other parts of the world.
"Anyone who does not want to see the Ukrainian crisis should refrain from doing things which may lead the other parts of the world into a crisis like this," Zhang told reporters.
"As the Chinese saying goes: If you do not like it, do not impose it against the others."
