There could be as many as 23,000 Islamic extremeists in the UK… but they cannot all be monitored.
The figure is more than six times the figure previously released by the Home Office at the time of last month's Manchester bombing.
Then, it was revealed the security services had 500 live investigations into more than 3,000 suspected radical jihadis, including about 400 people who have remarkably been allowed to return to our shores after fighting with terror group Islamic State (ISIS) in Syria.
But, it has since emerged a further 20,000 radical Islamists have been considered a "person of interest" to the security services at anyone time, according to a security services source.
It is not clear over which timescale the figure relates to.
But, the news has led to fears there are simply not enough resources for our security forces and counter terror police to monitor all their activities at anyone time.
A Whitehall source said 18 terror plots in the UK have been foiled since 2013, including five since the London terror attack in March.
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Armed police responding to the incident at London Bridge
But round-the-clock monitoring of terror suspects is only possible in a handful of cases due to limited resources, the priority of which are determined on factors including credibility of intelligence and the apparent seriousness of the individual's intent, according to the source.
But, Salman Abedi, who killed 22 people and injured 116 others at the Ariana Grande concert at Manchester Arena on May 22 was a "person of interest" who slipped the net and was allowed to travel from the UK to Libya and return in the run up to the atrocity.
Investigations are underway into how this was able to happen.
And, it has emerged at least one of the three dead terrorists involved in Saturday's London Bridge attack was on the radar over radical views.
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Security minister Ben Wallace said all 23,000 people were "in the mix and had to be looked at", but could not say how this was possible.
He said: “All of that is predominately underpinned by intelligence, which as I'm sure you will understand and the courts certainly understand, unfortunately the hardest part is we've got to get convert intelligence into evidence if we actually want to deprive people of their liberty or take certain steps.”
After the Westminster and Manchester strikes, Express.co.uk exclusively revealed powers available to the authorities to question terror suspects were used less last year than in 2015.
This comes at the same time the number of people arrested on suspicion of plotting terror strikes on British soil has grown by eight per cent.
In 2016 there were 260 terror-related arrests, a drop of eight per cent on the 282 people held in 2015.
The figures were revealed in a quarterly Home Office report on the use of police powers under the Terrorism Act 2000, which was released in March.
Last year 13 per cent (35) of arrests were of people suspected of plotting terror attacks on British soil, a sharp rise on the five per cent (15) of such arrests in 2015.
The report said: "Of the 260 arrests for terrorism-related offences in the year ending 31 December 2016, 96 had resulted in a charge by January 16 2017 (79 terrorism-related and 17 non-terrorism-related).
"In 68 cases, the individual had been released without charge; in a further 90 cases, the arrestee was released on bail pending further investigation."
The use of a power for officers to question people entering or leaving the country at ports, airports, international rail stations and in border areas fell dramatically by 30 per cent in 2016 compared to 2015.
The hardest part is we've got to convert intelligence into evidence if we actually want to deprive people of their liberty or take certain steps.
Security minister Ben Wallace
In 2015, the power was used around 27,800 times, but this fell to 19,355 last year.
Under the power, people can be detained for up to nine hours, with DNA samples and fingerprints taken, without the need for any reasonable suspicion.
The report said: "When necessary they may also detain and search individuals. "The aim is to determine whether that person appears to be someone who is or has been concerned in the commission, preparation or instigation of acts of terrorism.
"In the year ending 31 December 2016, a total of 19,355 persons were examined under this power in Great Britain, a fall of 30 per cent on the previous year.
"This has continued the decline in the use of the power in recent years."
It does not explain in the report why the number has dropped, but there has been pressure from groups like Stop Watch, which argues they infringe the human rights of Muslims and other communities.
But, independent peer Lord Carlile called for the reintroduction of tougher control orders banned in 2012 over human right fears.
He said it was a “grave mistake” to abolish the orders that allow security services to monitor terror suspects.
The QC said the sweeping powers – which restricted suspects’ movement, use of phones and computers – "may have saved dozens of lives" between 2005 and 2011.
Last night UKIP leader Paul Nuttall called for their return saying "the safety of UK citizens is more important than the human rights of jihadis".
He even proposed the possible use of waterboarding to interrogate suspects saying with threat levels as they are he could not "take anything off the table".
Manchester bombing: Investigation so far Wed, May 24, 2017
The investigation so far into the Manchester bomb attack that killed at least 22 people, with more than 60 injured after Salman Abedi detonated a bomb in the Manchester Arena, at 10:30pm Monday, 22 May 2017
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Handout photo courtesy of Alex Finnie, of men wearing body armour during a raid of a block of flats in Blackley, north Manchester, following the attack on Manchester Arena
The quarterly report also showed the number of people detained after being examined under the powers fell in 2016 by 16 per cent.
There were 1,828 detained in 2015 and 1,539 last year.
The amount of stop and searches of suspected terrorists anywhere in the UK, when reasonable suspicion does exist, fell by seven per cent in 2016.
There were 483 uses of the power in 2016, a fall from the 521 in 2015.
The report said a high number of arrests and use of stop and search in the last quarter of 2015 was the reason for the decline.
It said: "This fall is driven by a particularly large number in the October to December 2015 quarter in the previous year, rather than a particularly low number in the most recent quarters."
Last month also revealed how the Home Office was refusing to say if a single returning ISIS fighter had been questioned or detained.
It later emerged powers introduced by former Prime Minister David Cameron to ban British jihadists from returning to the country after fighting with Islamic State have only been used once.
In November 2015 MI5 boss Andrew Parker spoke of the increased threat of the Syrian crisis.
He said: "It represents a threat which is continuing to grow, largely because of the situation in Syria and how that affects our security."