Speaking at a conference in London, Gilles de Kerchove, the EU's counterterrorism co-ordinator, said terrorist groups such as al-Qaeda and Isis will soon turn to constructing biological weapons in their own homes.
He claimed the next step could be "a similar article on how to process a virus in your mum's kitchen". He also warned the terrorists could use drones to spread such infections.
Underlining the threat of Isis's online presence, Theresa May's deputy national security adviser, Patrick McGuinness, told the conference: The online space is the frontline.
"Until Isis cannot occupy space online freely, we will not be safe...The speed with which people are brought to violence is almost too fast to catch."
"These companies have got it in them to resolve this issue very substantially and free us largely in this space."