Police and spies are to stage a mock ‘marauding terrorist attack’ on the streets of London today to test how emergency services deal with a deadly atrocity in the wake of the ISIS killings in Tunisia.

In the biggest exercise of its kind ever staged, Operation Strong Tower will be a ‘noisy and visible’ practice run for more than 1,000 Met police, armed forces, transport workers and Whitehall officials.

Details of the plan were made public to prevent panic, as police chiefs said it would include ‘live play’ of a terrorist attack at a tube station, days before London marks 10 years since the 7/7 bombings which killed 52 people.

The extraordinary public display of how the UK’s security services would respond to an extremist’s deadly assault comes as efforts continue to identify British holidaymakers killed by an ISIS gunman on Friday.