Counter-terrorism investigators say prime concern is the hundreds of Britons who trained with Isis in Syria then returned

An Isis fighter in Raqqa Images, from Isis propaganda footage.
An Islamic State fighter in Raqqa, Syria, from Isis propaganda footage. Photograph: Screengrab

Islamic State commanders in Syria have communicated with jihadis attempting to stage terrorist attacks in Britain in the past year, counter-terrorism investigators believe.

The intention was to add Britain to the list of western countries hit in the past 12 months. An attack in 2017 is still deemed highly likely and the severe terrorist threat level is expected to remain.

A source said: “There is an assumption that it won’t abate in 2017. People do not believe that it is going to get better.”

In the last 30 months, since the rise of Isis in 2014, 11 Islamist attack plots on British soil have been identified and disrupted. Authorities in Britain believe there were four plots in 2016 aimed at inflicting mass casualties, all of which were shut down after arrests were made. Some are believed to have been inspired by Islamist propaganda. But there have been plots with direct involvement from Isis military planners in Syria, communicating with terrorists trying to attack the UK, investigators believe.

Extra security measures were introduced in Britain after a truck attack on a Christmas market in Berlin killed 12 on Monday.

Police and the National Crime Agency have launched their biggest effort to take guns off the street before terrorists can get hold of them. On Friday the NCA said the effort to capture illegal firearms had “never been a more significant priority”.

Counter-terrorism detectives and MI5 are experiencing intense workloads as they track terrorist activity.

More than 850 Britons are believed to have travelled to Syria after Isis declared a caliphate in August 2014. About 15% were killed while overseas, mostly while fighting for the terrorist group. An estimated 350 remain abroad.

Of the remainder, the prime concern for counter-terrorism investigators is the estimated 300 people who went to train with Isis in Syria then returned to Britain.

The rate of Britons going to join Isis in Syria has slowed, possibly because awareness campaigns, or that most of those minded to join have already done so. The rate of Britons returning from Syria has also slowed.

Investigators believe that a former commander in charge of planning attacks on Europe, Abdelhamid Abaaoud, sent a fellow terrorist to British cities to plan an attack.

Mohamed Abrini visited locations in Britain in 2015 including shopping centres in Birmingham and the Old Trafford stadium in Manchester. Abrini, since arrested over the attack on Brussels airport in March, claimed he was in the UK to collect money.

Raffaello Pantucci, from the Royal United Services Institute, said: “Isis is very interested in hitting the UK, but it is harder, so they are going where it is easier. We’re likely to see a continuing level of threat we’ve seen in the last couple of years.”

The threat has been mitigated by the fact Britain does not share extensive land borders with Europe and has tight gun laws. But throughout the past year the increasing availability of guns has been a growing concern.

The rise of Isis and its threats to attack the west led Britain in 2014 to raise its terrorist threat level. “Severe” means the joint terrorism analysis centre believes an attack is highly likely.