Andrew Parker said ISIS still aspires to direct terrorist atrocities despite territorial losses, in the wake of 45 attacks across Europe since 2016.
Thanking European security agencies for their support in the investigation following the Manchester bombing nearly a year ago, the intelligence chief disclosed that MI5 has thwarted 12 plots alongside police since the Westminster atrocity in March 2017.
This brings the total number of disrupted attacks since 2013 to 25.
Mr Parker said that he was ‘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’.
The director general of the Security Service added that ‘European intelligence cooperation today is simply unrecognisable to what it looked like five years ago’.
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Mr Parker, who has a 35-year career in intelligence, said: ‘In today’s uncertain world we need that shared strength more than ever.’
In his first public comments on the Salisbury nerve agent attack, Mr Parker described them as a ‘deliberate and targeted’ act.
He accused the Kremlin of ‘flagrant breaches of international rules’ and warned that the Russian government is pursuing an agenda through ‘aggressive and pernicious actions’ by its military and intelligence services.
Referencing the unprecedented level of disinformation following the attack, Mr Parker set out the need ‘to shine a light through the fog of lies, half-truths and obfuscation that pours out of their propaganda machine’.
The attack on former double agent Mr Skripal, 66, and his daughter Yulia, 33, sparked a massive chemical alert.
A huge investigation to identify the would-be assassin or assassins is ongoing.
The British Government has pointed the finger at Russia, but Moscow has repeatedly denied responsibility.
Mr Skripal remains in hospital, while his daughter was released to a secure location last month.