The UK government's secretive National Cyber Force has launched operations against militants, state-backed disinformation campaigns and attempts to interfere in elections, the Government Communications Headquarters spy agency said on Tuesday.
The NCF is a hacking unit set up in 2020 that consists of spies and defence officials from Britain's armed forces and GCHQ.
In a rare statement about its work, GCHQ said the attacks had taken place over the past three years.
"In an increasingly volatile and interconnected world, to be a truly responsible cyber power, nations must be able to contest and compete with adversaries in cyberspace," GCHQ director Jeremy Fleming said.
The statement was published alongside a 28-page paper designed "to illustrate aspects of how the UK is being a responsible cyber power".
It did not elaborate on the specifics of those operations.
Britain's position as a leading hacking power competing alongside the US with countries including Russia, China and Iran has been long known but rarely acknowledged.
In 2018, GCHQ revealed a series of attacks it had conducted against ISIS. Since its establishment, Britain has given little information about the activities of the NCF.
Since it began, the NCF has used its offensive cyber capabilities to protect overseas military bases and "disrupt terrorist groups", the headquarters said.
British government hackers have also countered state disinformation campaigns and worked to "reduce the threat of external interference in democratic elections", it said.
The paper accompanying the statement did not say which disinformation-spreading states British hackers had worked to counter.
But it noted that "countries such as Russia and Iran routinely carry out cyber operations of different kinds in order to spread disinformation".
"The NCF's work is covert and we therefore do not reveal details of individual operations," the GCHQ said.
"Indeed, the intent is sometimes that adversaries do not realise that the effects they are experiencing are the result of a cyber operation.
"This ambiguity can help to amplify the cognitive effect".
The statement named James Babbage, a GCHQ intelligence officer of 30 years, as commander of the NCF.
