LONDON - Russia is committing “flagrant breaches of international rules”, the head of Britain’s security service will claim Monday in which he will also stress the importance of post-Brexit security ties.

MI5 chief Andrew Parker will say 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 will say 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 will also condemn 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.

The need “to shine a light through the fog of lies, half-truths and obfuscation that pours out of their propaganda machine” will be set out in his remarks, the first ever made outside Britain by a serving head of MI5.

Skripal, 66, remains in hospital following the attack in the Wiltshire city on March 4. His daughter Yulia, 33, and the British policeman have both been discharged from hospital.

A huge investigation to identify the would-be assassin or assassins is ongoing.

Parker will praise the international response to Salisbury, in which 18 of the 28 European Union countries agreed to support the UK in expelling scores of Russian diplomats.

Also thanking European security agencies for their support in the investigation following the Manchester bombing which claimed 22 lives nearly one year ago, the intelligence chief will disclose that MI5 and police have thwarted 12 plots since the Westminster attack of March 2017 which claimed five lives.

This brings the total number of disrupted attacks since 2013 to 25.

Parker will say he is “confident about our ability to tackle these threats, because of the strength and resilience of our democratic systems, the resilience of our societies and the values we share with our European partners”.

“European intelligence cooperation today is simply unrecognisable to what it looked like five years ago,” he will say, adding: “In today’s uncertain world we need that shared strength more than ever.”

But his comments will be made the same day as the British House of Lords warns the country needs to set out plans for post-Brexit security cooperation with the EU.

“We are concerned that the government has yet to explain how its high level aspirations could be put into practice,” the Lords EU External Affairs Sub-Committee’s study published Monday said.