Police chiefs have ordered a major surge in the number of armed officers on duty in the wake of the Westminster attack.
Hundreds of extra firearms personnel are on hand around the country as forces mount a large-scale security operation following the outrage.
Scotland Yard head of counter-terrorism Mark Rowley said the service will sustain an "enhanced" armed and unarmed presence over the next few days.
In London the number of armed officers remains at near double strength, while there are up to a third more on duty in other parts of the UK.
Forces outside London have stepped up their firearms responses in light of the atrocity in the capital.
Essex Police has implemented additional armed patrols across the county, while West Yorkshire has implemented an "uplift" in the capability of armed response vehicles.
Last year Scotland Yard announced plans to increase the number of firearms officers on hand to protect the capital by 600 in the wake of the Paris terrorist attacks in November 2015.
Nationally the armed policing strength is being boosted by 1,500 personnel.
The most recent figures available showed there were 5,639 authorised firearms officers in forces across England and Wales as of the end of March last year.
This was a slight fall on the previous year and meant the number had dwindled by more than 1,000 in five years, but the figures did not include the major drive to boost the armed capacity.
Most police personnel in the UK are unarmed, setting the country apart from many other nations around the world.
But the question of whether officers should carry guns as a matter of course has been debated for decades, and the issue has come under scrutiny again after recent attacks in Europe.
Research has suggested the majority of police are opposed to any change in approach, but surveys of members of the public have proved less conclusive.