Britain's counter-terrorism agencies are working at an "extraordinary" pace to try and contain the threat, a senior officer has said.

Around 500 investigations are being carried out into 3,000 people at any one time by MI5 and police.

The agencies are also working to prevent threats of copycat strikes following the three recent atrocities in the UK.

Commander Dean Haydon, Head of the Metropolitan Police's Counter Terrorism Command said security agencies are working in "unprecedented times".

"We are doing everything we can alongside the security services to stop would-be attackers, to keep the public safe.

"The tempo of our work is extraordinary. We are running around 500 investigations into 3,000 people at any one time. That's not just in London, that's nationally.

"In these current times, we also need to watch out for those who may be inspired by these recent attacks."

Since mid-2013 security services have foiled 18 plots, including five since the Westminster attack in March.

Security measures have been stepped up across London including protective barriers being installed at eight bridges around the capital.

Police are considering the use of barriers and other security measures at crowded and iconic locations across London and continue to review security at events over the summer.

They are also continuing to work with haulage and vehicle rental companies to develop a greater understanding of potential risks and suspicious activity they should look out for.