Boris Johnson today blamed American whistleblower Edward Snowden for teaching terrorists how to avoid being caught.
And he said he thought a Paris-style attack in London was unlikely but impossible to rule out.
Snowden is a former CIA employee who leaked details of the US government's secret internet surveillance programmes.
"To some people the whistleblower Edward Snowden is a hero; not to me. It is pretty clear that his beanspilling has taught some of the nastiest people on the planet how to avoid being caught," he added.