The threat to the UK from returning Islamic State jihadists is not as significant as police and intelligence agencies had originally feared, security sources have told Sky News.
However, the threat from homegrown extremists has reached unprecedented levels, with significant numbers of people being directly encouraged or instructed to carry out attacks.
One source said: The threat picture has diversified enormously in recent months and is likely to remain severe for the foreseeable future.
As IS comes under pressure in Raqqa, as they did in Mosul, the impetus to go and join them has started to dissipate, but their message remains potent for those willing to listen.
Part of the problem is that propaganda had democratised the threat so that self-starters and lone actors can view material that is pumped out encouraging them to get out with knives or vehicles to launch these low-tech attacks.
More than 800 Britons have travelled to Syria and Iraq to fight with ISIS.
Police and security services feared that, as the terror group crumbled in the Middle East, hundreds would find their way back home as fully-trained killers.
But the actual number of jihadist returners is significantly less than they expected.
Around 350 British citizens have already returned to the UK. Some have been prosecuted, but police believe most of the others were probably not directly associated with IS.
Sky News has been told up to 200 British fighters are believed to have been killed in the Middle East, with the remainder either unwilling or too afraid to return for fear of prosecution.
The source said: We now think that fewer are likely to return from Syria and Iraq than had previously been feared, partly because they are likely to have been killed in the fighting as it escalates and partly because those that survive are likely to seek refuge in other countries.
The threat from those who have never left the UK for Syria and Iraq is the predominant threat – they are still highly motivated and increasingly they are taking direction from individuals in the Middle East.
Authorities tell us they are monitoring around 100 British jihadists who they believe may return, so that threat has not gone away.
Security minister Ben Wallace MP said: I think what has happened is IS has very cleverly switched tactics.
They have realised that they cannot get the supply of fighters into Syria and Iraq and indeed they have been defeated, predominantly in Iraq.
But at the same time, IS think they need to spread terror, so what we have seen them do is use more inspiration, use more of the internet to radicalise those people to take action at home, and do whatever they can to kill people.
Intelligence agencies believe that advances in communications technology – such as encrypted chat websites and apps – are allowing those in the heart of the warzone in the Middle East to directly encourage radicalised individuals at home.
The UK’s independent reviewer of terrorism legislation, Max Hill QC, told Sky News: It is not always possible to find a back door into device to device encrypted communication, but there are ways in which the tech companies, can and should be co-operating with law enforcement.
The template for increased terror activity in the West was set more than a decade ago, with the dismantling of al Qaeda.
Under intense pressure from coalition strikes, the group urged followers to launch attacks in London, Madrid and in other western cities.
But terrorism experts say the authorities have an ever greater challenge this time around.
Raffaello Pantucci, director of International Security Studies at the Royal United Services Institute, said: The difficulty for the security services is that they have now got a threat picture that is now so diffuse; it’s incredibly easy for anyone to pick up a knife, get into a car, and commit something, which from their perspective, they would consider a terrorist act in advance of their cause.
UK authorities believe there are as many as 3,500 individuals here who have an Islamist extremist mindset and may be susceptible to messages encouraging them to turn to violence.
The terror threat the UK faces, we are told, is not about to go away anytime soon.