Russia has warned Britain to “consider the consequences” of mounting a retaliatory cyber strike after the Salisbury spy poisoning.
In a fresh sign of the escalating diplomatic tension sparked by the case, the Russian Embassy cautioned against “such a reckless move”.
Theresa May has set Moscow a deadline of midnight on Tuesday to explain how a nerve agent was deployed against Sergei Skripal and his daughter.
If there is no credible response from the Kremlin, Mrs May has pledged to set out a “full range” of measures to be taken in response.
The Government has not publicly disclosed the options under consideration but reports on Tuesday suggested one possibility was a cyber counter-attack.
Responding to the speculation, the Russian Embassy in the UK said: “Statements by a number of MPs, ‘Whitehall sources’ and ‘experts’ regarding a possible ‘deployment’ of ‘offensive cyber-capabilities’ cause serious concern.
“Not only is Russia groundlessly and provocatively accused of the Salisbury incident, but apparently, plans are being developed in the UK to strike Russia with cyber weapons.
“Judging by the statements of the Prime Minister, such a decision can be taken at tomorrow’s meeting of the National Security Council.
“We invite the British side to once again consider the consequences of such a reckless move.”
Britain’s capacity in the area of “offensive cyber” was summarised in December by the Commons Intelligence and Security Committee, which has access to high-level security figures and highly classified material.
Offensive cyber covers a range of measures including possible retaliation after a cyber attack and capabilities to attack wider systems or infrastructure, according to the committee’s annual report.
It added: “Both GCHQ and MoD (Ministry of Defence) are responsible for developing these capabilities for the UK and this involves skills and techniques across a range of technical work in each organisation.”
Offensive cyber work is “broad” and includes: The development of computer code or “hacking” tools; intelligence development, for example through interception; and “delivery” – either by network or physical access.
Deployment can take place through a specific device or an adversary’s computer network, according to the report.
In 2015, then chancellor George Osborne said: “We are building our own offensive cyber capability – a dedicated ability to counter-attack in cyberspace.”
The committee’s report quoted evidence from GCHQ about the programme.
The passage was partly redacted but the intelligence agency referred to a “full spectrum of capabilities” from “tactical stuff” through to “the high end of counter state offensive cyber capabilities which might never be used but are the sort [of] high-end deterrents”.