A suspected jihadist carrying a rucksack packed with knives was heading towards Downing Street when he was dramatically arrested by armed police, security sources have told The Telegraph.

The ‘lone wolf’ suspect had been under close surveillance when counter-terrorism officers ordered his immediate detention as he came within 300 yards of the gates of the Prime Minister’s residence.

It is understood that the suspect’s family had become concerned about his behaviour and reported him to the authorities several weeks ago. Investigators believe he was about to launch an attack.