Russia is committing "flagrant breaches of international rules", the head of Britain's security service claimed yesterday.

MI5 chief Andrew Parker said in a landmark speech in Berlin that a March nerve agent attack on Russian nationals Sergei and Yulia Skripal, which also injured a police officer, was a "deliberate and targeted malign activity" on British soil.

Parker, who has a 35-year career in intelligence said the attack is evidence of Russia pursuing an agenda through "aggressive and pernicious actions" and risks making the country a "more isolated pariah".

Russia has denied any involvement in the attack and has challenged Britain to reveal evidence to back up its claim that the Russian state was likely behind it.

Parker also condemned Kremlin disinformation following the attack -- the first use of a nerve agent in Europe since the Second World War -- in his speech before an audience of security chiefs in Berlin.