Britain is "closely monitoring" reports that China tested a nuclear-capable hypersonic missile earlier this year, according to Downing Street.
At the weekend, the Financial Times newspaper reported that China's military launched a hypersonic glide vehicle which flew through low-orbit space before descending towards its target.
Despite reports it had missed by about "two dozen miles", the paper said it demonstrated that Beijing's progress on developing hypersonic weapons was more advanced than US intelligence previously believed.
China's foreign ministry spokesman Zhao Lijian said on Monday that a spacecraft, not a missile, had been launched in the summer.
Downing Street said the British Government was keeping a "close watch" on developments.
Watch: Defence expert Dr James Bosbotinis tells Forces News the reported missile test will not have surprised many.
The Prime Minister's official spokesman said it was important for all powers with a nuclear weapons arsenal to behave "responsibly".
He added: "We are closely monitoring the situation and as ever are clear that all states with nuclear capability must act responsibly within the international system," the spokesman said.
"We have seen what has been reported.
"We will keep a close watch on this development but it is important that those with these capabilities behave responsibly."
Asked if the UK accepted Beijing's explanation that it was a spacecraft, not a weapons system, the spokesman said: "At this stage, I am not able to pass judgment on that without further information."