The threat from the Islamic State "movement" will persist for years to come despite its territorial losses in Iraq and Syria, the head of MI5 has warned.
Andrew Parker said the group - which he referred to by the name Daesh - is "seeking to regroup" after coming under sustained military pressure.
In a speech in Berlin, he said Europe faces an "intense, unrelenting and multi-dimensional terrorist threat".
IS continues to pose the most acute threat, but al Qaida and other Islamist terrorist groups "haven't gone away", the Director General of the domestic security agency said.
He added that alongside police, MI5 is also actively "monitoring the trajectory" of extreme right-wing terrorism.
The "unprecedented tempo" of attack planning in Britain shows no signs of abating, Mr Parker said.
He reported that since the Westminster atrocity in March last year, police and security services have foiled 12 Islamist terror plots.
Mr Parker said: "This upshift in threat is of course driven by Daesh's murderous ideology.
"Whilst Daesh has now lost its false caliphate in its strongholds in Syria and Iraq, tackling the group as a movement will require sustained international focus for years to come. They are seeking to regroup and the threat seems likely to persist."
He described the terror threat as three dimensional, with plots germinating at home, abroad and online.
"Daesh's twisted ideology continues to influence vulnerable and violent individuals across Europe and beyond to use crude but deadly methods to kill, from stabbings to vehicle attacks, from bullets to bombs, from hard to soft targets," Mr Parker said.
He also set out his hope that Britain and the EU can strike a "comprehensive and enduring" agreement on security co-operation after Brexit.
Mr Parker said MI5, MI6 and GCHQ are "absolutely committed" to continuing to develop their contribution to collective security.
"That is what the threats require," he said. "It is an unconditional commitment. We must not risk the loss of mutual capability or weakening of collective effort across Europe."