Manchester bomber Salman Abedi was a "former subject of interest" to the security services whose risk "remained subject to review", a Whitehall source has revealed.
Details of the intelligence agencies' knowledge of Abedi came as police hunting the network behind his attack said they had made "significant" arrests and seized "very important" items in raids linked to the investigation.
The senior source revealed that 18 plots had been foiled since 2013 in Britain. Those included five since the Westminster attack in March this year.
It is understood the scale of the threat being dealt with by counter-terror agencies is on an "unprecedented" scale.
The source said at any one time there could be 3,000 subjects of interest to MI5 and Abedi was part of a "larger pool" of people formerly classed in that category.
They said: "MI5 is managing around 500 active investigations, involving some 3,000 subjects of interest (SOIs) at any one time.
"Abedi was one of a larger pool of former SOIs whose risk remained subject to review by MI5 and its partners.
"Where former SOIs show sufficient risk of re-engaging in terrorism, MI5 can consider re-opening the investigation, but this process inevitably relies on difficult professional judgements based on partial information."
Meanwhile Greater Manchester Chief Constable Ian Hopkins hit out at the leaks of intelligence by US agencies to the media, which had caused "distress and upset" to the families of victims of the atrocity.
Theresa May said she will tell Donald Trump that intelligence shared with the US "must remain secure" in a sign of the UK authorities' anger at the repeated leaks of sensitive information to the media.
The terror threat level will remain at critical - meaning another attack is expected imminently - and in an indication of the level of counter-terrorism activity
In an update on the investigation, Mr Hopkins said: "I want to reassure people that the arrests that we have made are significant, and initial searches of premises have revealed items that we believe are very important to the investigation."
In a sign of the UK authorities' anger about the repeated leaks of information in the US it is understood that police have stopped sharing intelligence on the bombing with their US law enforcement counterparts following the leaks.
Mr Hopkins said the leaked photographs and information published by the New York Times had caused "much distress for families that are already suffering terribly with their loss".
Mrs May was meeting the US leader at a Nato summit in Brussels where she said she will "make clear to President Trump that intelligence which is shared between our law enforcement agencies must remain secure".
US reporters travelling with the president said Mr Trump declined to answer questions on intelligence sharing.
Mr Hopkins said there had been an "intense three days" for his officers and staff, along with the National Counter Terrorist Policing Network and UK intelligence services.
:: Two men were arrested by police in Greater Manchester overnight on Thursday, bringing the number in UK custody to eight.
:: One of the men was detained following searches of an address in the Withington area of the city, while another was arrested in a part of Greater Manchester that was not disclosed.
:: On Wednesday, a series of raids were executed across Manchester, Wigan and Nuneaton in Warwickshire, while relatives of Abedi were detained in Libya.
:: In the early hours of Thursday, counter-terror police carried out a controlled explosion at a property in the Moss Side area of Manchester, although no arrests were reported by police.
Mr Hopkins said searches in connection with the investigation would take "several days to complete"
The bomb detonated by Abedi targeted music fans - many of them young - at an Ariana Grande concert on Monday evening, killing 22.
Latest numbers released by NHS England show that as of 6am on Thursday:
:: 116 people received NHS inpatient care in relation to the Manchester incident;
:: 75 people have been admitted across eight hospitals. including 23 patients who are in critical care.
The nation fell silent at 11am to mark the atrocity.
Crowds gathered at well-known sites in the UK, including London's Parliament and Trafalgar Squares, and Manchester's Albert Square.
Acting American ambassador to the UK Lewis Lukens said the US was "determined" to identify and stop the leaks of information.
He told BBC Radio 4's World At One programme: "At all levels of government we have heard the message loud and clear from Her Majesty's Government and we agree with their concerns and we're determined to take action."
Mr Lukens said intelligence co-operation across the Atlantic "keeps both of our countries much safer" but said it was "up to the United Kingdom ultimately to determine how we work together on this".
"These leaks are terrible, and again let me just say in the strongest possible terms that we condemn them and we are determined to investigate and to bring appropriate action.
"We have had communications at the highest level of our government ... we are determined to identify these leaks and to stop them."
Arriving at the Nato summit in Brussels, the Prime Minister said the "special relationship" with the USA was "built on trust".
She said: "Part of that trust is knowing that intelligence can be shared confidently and I will be making clear to President Trump today that intelligence that is shared between law enforcement agencies must remain secure."
Home Secretary Amber Rudd said she was "confident" that the leaks would stop and the suspension of intelligence-sharing would be temporary.
She said: "Greater Manchester Police and the counter-terrorism police have taken a view. We hope that it is just going to be temporary, but it has been worrying, the leaks.
"But we are confident that will now end."