Britain's threat level has risen sharply

“The Joint Centre of Terrorism Analysis has changed the level of threat of terror in the United Kingdom, from essential to serious”, he wrote on Twitter.

This is in advance and is not based on any specific threat. The public must continue to remain vigilant and report to the police any suspicious activities”.

Under the government's rating system there, the new threat level means an attack is most likely to occur.

The change came after an armed person in Vienna, identified as a convicted jihadist, killed four people overnight.