Police may now have to shoot terrorists at the wheel of vehicles to stop them being used in attacks, the national lead for armed policing has said.
Officers were previously told not to shoot drivers of moving vehicles because of the additional dangers it posed, Simon Chesterman said.
He added officers now had ammunition to penetrate doors and windows.
Last month, Khalid Masood killed four people on London's Westminster Bridge when he mounted the pavement in a car and drove into pedestrians.
One of the biggest challenges previously faced by police was the risk of bullets bouncing off the glass of vehicles in such attacks, said Mr Chesterman, of the National Police Chiefs' Council.
"We've seen some very horrible and different tactics lately involving vehicles and lorries," he said.
"Within our policy, we used to talk about not shooting at a moving vehicle because of the danger we might cause if we fired at a driver.
"But if the vehicle is being used as a weapon in the first place, there aren't many tactics available in relation to stopping it, particularly a very large lorry.
"Driving a vehicle in front of it for example is not going to stop it. So you need to shoot the driver," he said.
As part of the change in policy, Mr Chesterman said the tactics of firearms officers were now "far more aggressive".
They are no longer being told to "locate, contain and neutralise" but to "locate and confront", he said.
He also announced that the number of authorised firearms officers available to be deployed across England and Wales is to reach 10,500 by next year - an increase of about 1,500.
In April 2018, there will be about 7,000 armed police from the 43 forces in England and Wales, and 3,500 from other forces, including the Civil Nuclear Constabulary.
The government is funding 1,000 extra armed officers, while forces are paying for 500 more.
Firearms training: Toughest school in Britain?
- Up to a quarter of police officers who want to join mobile firearms teams fail the demanding training carried out at a number of national training centres.
- It takes three months to train an officer to join armed response vehicles, the backbone of the UK's response to serious violent crime and terrorism
- Recruits need to prove they can safely handle guns, justify their decisions and, crucially, save lives - including suspects they have shot
- Officers can later apply for specialist and more demanding roles, such as specialist counter-terrorism teams
Mr Chesterman said the capability of armed officers was "phenomenally different" from previous years following investment in recruitment and training.
But he warned that concerns about the way police are treated after fatal shootings could act as a "tipping point" and discourage people from joining or staying on.