GETTY Up to 23,000 jihadis could be living in Britain, security officials say

FREE now SUBSCRIBE Invalid email Make the most of your money by signing up to our newsletter fornow We will use your email address only for sending you newsletters. Please see our Privacy Notice for details of your data protection rights.

Security services revealed the figure as they said up to 3,000 would-be terrorists are being investigated as “subjects of interest”. The announcement, unusual for security agencies such as MI5 and MI6, highlights the difficulties in investigating potential terror threats, according to insiders.

GETTY There will be a bolstered security presence across the UK this Bank Holiday

Amid a reported 500 active terror investigations, one security source told the Mail: “It is difficult because becoming aware that someone is an extremist sympathiser does not make them a live and present danger. “It is a balancing act – is the intelligence sufficient to push them up the list of priorities or should the focus be on someone else given the finite resources available?”

Who are the Manchester bombing victims? Thu, May 25, 2017 At least 22 people have died, with more than 60 injured Play slideshow PA 1 of 13 Sorrell Leczkowski, a 14-year-old schoolgirl from Leeds

The warning comes as extra police officers and soldiers are deployed to thousands of events across the UK this bank holiday weekend amid heightened terror fears. Sick terror group Islamic State, which claimed responsibility for Monday’s attack that killed 22, has called for “all-out war” against the west as the Islamic holy month of Ramadan begins.

But police officials have announced beefed-up security at events such as Saturday’s FA cup final, the premiership rugby final and the Great Manchester Run. And for the first time armed police will patrol Britain’s trains as soldiers guard the Houses of Parliament, Downing Street and Buckingham Palace.

Film premieres in London have also been scrapped in the wake of the horror bombing. The UK premiere of The Mummy at Leicester Square was scheduled to take place on Thursday but Universal Pictures said the screening has been cancelled out of respect to the victims.

GETTY NHS trauma centres have been told to prepare for another attack