David Otto, counter-terrorism and organised crime expert at Global Risk International, points to the return of ISIS fighters from Iraq and Syria.
He told Daily Star Online: “Most of these fighters have come back and governments haven’t really had a clue as to how to handle them.
“Some have returned without governments knowing, they are now like sleeper cells.
“It makes it even more dangerous because you never know when they can carry out a replica of the Lee Rigby murder.
“This is a huge concern, especially as most of these groups have taken the form of going underground – just coming out when it’s time to strike.”
The sleeper cells may carry out their attacks during Armed Forces week – which started on Monday – in a show of defiance against British troops.
Mr Otto added: “Events like that may be a call for sleeper cells to strike.
“Especially because the military has been responsible for the destruction of ISIS strongholds, the UK is part of that coalition.
“Events like that may be a call for sleeper cells to strike”David Otto
“This is the time when the security services have to pay more attention with these groups going underground.”
Rigby’s killers mowed him down in a car before hacking him to death close to his barracks, in what they said was a "revenge attack" for the killing of Muslims by British soldiers.
Back in April, one of them said he was desperate to leave the notorious HMP Wakefield, known as Monster Mansion.
Michael Adebowale, 37, told pals he wanted a transfer to the cushier Kirikiri Prison, which has a VIP section for prisoners who can afford to pay for a better cell.
Kirikiri is a maximum-security prison in Lagos State, home to some 5,000 inmates