The head of GCHQ has vowed to retaliate against the “brazen Kremlin” for Russia’s nerve agent attack on Salisbury.

Jeremy Fleming, the director of GCHQ, said the Government’s top secret cyber intelligence agency would “deploy the full range of tools” to counter the threat posed by Vladimir Putin’s regime.

Mr Fleming’s intervention - part of the UK’s coordinated response to the Novichok attack orchestrated by Russian military intelligence - further ratcheted up the pressure on the Kremlin.

Whitehall sources said Britain possessed the “offensive cyber capability” to target the GRU and individuals linked to it.

Theresa May said on Wednesday the GRU had dispatched two senior officers to the UK to carry out the assassination on March 4 of Sergei Skripal in the Wiltshire city.