Britain's security services have foiled 12 attempted terrorist attacks on UK soil since June 2013, the head of MI5 has said.
The internal security service's director general Andrew Parker announced the figure in a talk to the Royal Society's fourth annual diversity conference.
He said that terrorism - particularly from so-called Islamic State - posed the most visible security threat to Britain today.
Speaking after the event, he warned that security forces would not be able to stop every planned terror attack.
ISIL is an enduring threat, here to stay, and is at least a generational challenge. MI5 and the intelligence agencies have good defences because of the investment made in our capabilities. We will find and stop most attempts to attack us, but not all.
Mr Parker said that MI5 was developing increasingly sophisticated techniques in IT, science and technology to identify and counter threats in a rapidly changing world.
He said that the service needed more highly skilled staff such as scientists, technologists, engineers and mathematicians to continue their work.
"My experience is that for all the challenges MI5 faces, we are stronger and better placed to rise to them with the richest mix of talents we can find," he added.
"It's what makes it less likely people will be killed by terrorism or our secrets will be stolen from under our noses."