Armed police, emergency services and soldiers around Aldwych station for mock terror attack testing London response to Tunisia-style shooting

A major counter-terrorism exercise is being carried out to ensure emergency services are ready "should the worst happen", Met Police Commissioner Sir Bernard Hogan Howe has said.

Around 1,000 officers, emergency services, soldiers and intelligence officials are taking part in the operation.

Sir Bernard said the rate of arrests for terror offences has increased in the past year.

He said: “It is clear during that time that we've disrupted some very serious plots and some of those cases are going to the courts.

"We intend first of all to stop them from getting to attack. But should we not stop the terrorists in their planning, it's essential we disrupt them in any of the attacks that may take place.”

Preparations for the exercise began around six months ago and events such as the Charlie Hebdo attack in Paris and the hostage siege in Sydney influenced the test.

The Mumbai attack in 2008 also featured in the planning.

Scotland Yard stressed the test was in no way connected or prompted by the massacre in Tunisia last week.

Sir Bernard said the exercise "will test our people in how to respond to a terrorist threat and we will learn from the mistakes that we are bound to make.

"It's best we make them in an environment where we don't have terrorists, than make those mistakes when we do."

The two-day exercise will see a test of the emergency service response to an attack at the disused Tube station at Aldwych.

The vast majority of police officers involved have no idea about the venues or what is planned. Only around 12 people knew specific details of in advance.

Among those taking part are all of London's emergency services, Greater London Authority, Transport for London, Home Office, Cabinet Office, Ministry of Defence, the Department of Health and NHS England.

Surrey Street will be shut to minimise disruption.