Britain has faced four major terrorist plots in the past year, three of them in the past few months alone, the head of MI5 has disclosed as he warned that the lethal threat from Islamist extremists, including those home-grown, has continued to grow at an unprecedented rate.
The security service is currently engaged in operations to counter attempts to stage atrocities, or "spectaculars": some of them involve jihadists from the UK who have been trained in Syria and Iraq to carry out murderous attacks like those which have claimed 13 lives in France in the past 24 hours.
"We know, for example, that a group of core al-Qaeda terrorists in Syria is planning mass-casualty attacks against the West," Andrew Parker said during a speech at MI5 headquarters for members of the Royal United Services Institute, a military think tank.
"It was primarily the rising threat from Syria that led the Joint Terrorism Analysis Centre to raise the threat level for international terrorism from 'substantial' to 'severe', an evidence-based judgment that an attack on the UK is highly likely.
"My sharpest concern as the Director-General of MI5 is the growing gap between the increasingly challenging threat and the decreasing availability of capabilities to address it," added Mr Parker.