The HS2 high-speed rail project has come under renewed attack by the Commons public accounts committee, who have accused the Department of Transport of failing to present a "convincing strategic case" for the £50 billion project.
The public spending watchdog raised a number of questions about the apparent benefits and warned the costs were spiralling.
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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.