According to The Times UK, a "dirty bomb", (a conventional explosive with waste radiological materials mixed in to cause panic and terror) or a biological weapon may be detonated within the UK by the Islamic State:
Businesses have been warned that Isis fighters returning to Britain from Syria and Iraq will seek to carry out a mass-casualty chemical or radiological “dirty bomb” attack, possibly using a drone.
Law enforcement agencies believe hardened Isis recruits are planning to import terrorist tactics perfected on the battlefields of the Middle East, including the deployment of chlorine and mustard gas, to the UK.
According to sources within Bulgaria, a "huge amount of explosives and weapons was caught two days ago" in Bulgaria, and there was no doubt they were intended for the West.
Bulgarian medical and security experts discuss the biological weapons aspect of this likelihood at just after the five minute mark: