The head of MI5 today warned Britain is facing the biggest terror threat of his 34 year career - as he said tech giants have an 'ethical responsibility' to crack down on extremists.

Andrew Parker said extremists are mounting deadly terror attacks with just a few days of planning as the UK sees a 'dramatic' jump in the scale and pace of the threat.

In his annual 'state of the union' assessment of the threat facing the UK, he said extremists are exploiting 'safe spaces' online, hindering intelligence efforts to root them out. 

And he issued a fresh challenge to technology firms, saying they have an 'ethical responsibility' to help governments confront the threat.

His stark warning comes after Britain was hit by five terror attacks this year - killing dozens and injuring hundreds more. In his first substantial remarks since the wave of carnage, this year, Mr Parker detailed the 'acute and enduring' challenge posed by Islamist terrorism. 

Twenty plots have been foiled over the past four years, including seven in the last seven months.

Speaking in central London today, he said: 'We've seen a dramatic up-shift in threat this year.

'Today there is more terrorist activity, it's coming at us more quickly and it can be harder to detect.'

Security services and police are operating at a scale which is 'greater than ever before', he said. They are probing well over 500 live operations involving around 3,000 individuals, he revealed.

And the top spy also warned that as Islamic State - also known as ISIS or Daesh - are on the run in Syria, British intelligence services have to assess the risk posed by those fleeing the war in the Middle East.