A 26-year-old soldier has died during a training exercise in North Yorkshire, the Ministry of Defence has confirmed.
It has been reported that the soldier from the 1st Battalion Merican Regiment was taking part in a three-mile run at the Catterick Garrison training area when he collapsed.
More top news
The threat level has been raised to critical by MI5's Counter-Terrorism Analysis Centre.
Prime Minister Theresa May announced the decision from Downing Street.
A man, apparently armed with a small axe and a large stick, was detained close to the event and police cleared the area as a precaution.