British authorities have raised the current threat level for international terrorism from severe to critical, meaning an attack is "expected imminently."

Prime Minister Theresa May announced in a statement that the threat level had been raised after the Parsons Green tube bomb attack in London.

It means that military personnel will replace police officers on guard at certain protected sites which are not accessible to the public. "The public will see more armed police on the transport network and on our streets providing extra protection," she added.