Police forces in England and Wales are to train an extra 1,500 firearms officers to help protect the public from terrorism.
Many will be deployed in rapid-reaction teams, which will be on patrol and ready to react round the clock.
There will also be more counter-terrorism teams outside London and 40 new armed response vehicles.
The move will reverse a drop in the number of firearms officers from nearly 7,000 in 2009/10 to 5,875 in 2013/14.
Downing Street said the increased network of armed police units would complement military contingency plans already in place to deploy up to 10,000 troops in the event of a terror attack.
Most of the new officers will be funded by the government and will be trained within the next two years.
Last year, Prime Minister David Cameron set aside £143m over five years to boost the UK's armed response capability.
Announcing the latest initiative during a visit to the US, Mr Cameron said: "Our police and intelligence agencies work round the clock to keep us safe and it is absolutely vital that we support them with the right resources and kit.
"After the terrorist attacks in France last year, we decided to look at whether there was more we could do to protect people from the type of terrorist threat we now face.
"That's why we are increasing the number of specially trained armed officers up and down the country to make sure the police have greater capability to respond swiftly and effectively should they need to do so."
National Police Chiefs' Council lead for Armed Policing, Deputy Chief Constable Simon Chesterman said: "Over recent years we have significantly enhanced the training, tactics and weaponry of armed officers to ensure that they are capable of dealing with all types of terrorist attacks.
"We have also delivered enhanced capability across all emergency services to deliver an effective joint response. This additional uplift will ensure we are in an even stronger position to respond quickly and effectively to protect the public."
Some officers have cautioned that colleagues may fear facing criminal charges if they open fire.
Mark Williams, chief executive of the Police Firearms Officers Association, said the culture within the Independent Police Complaints Commission (IPCC) should change so officers were not automatically treated as "suspects" in the event of shooting incidents.
IPCC deputy chair Sarah Green said firearms officers worked in "challenging circumstances" but it was "right that police shootings resulting in death or serious injury are independently investigated".
Last year, police chiefs warned the home secretary about the shortage of 24/7 armed police cover outside London, saying it left regional cities vulnerable to attack.
The number of authorised firearms officers was more than 40% lower in both Nottinghamshire and Warwickshire in 2013/14 than it had been in 2008/09.
Numbers in London were down by more than 20%.
Central funding will pay for 1,000 extra firearms officers across England and Wales, including 600 already announced by the Metropolitan Police to be based in London and 400 in the rest of England and Wales.
A further 500 officers will be funded by the forces themselves.
An additional 40 armed response vehicles and teams are due to be "up and running" within the next 12 months, bringing the total number across the country to 150. London will see its fleet doubled.
The vehicles are adapted so the armed officers can be deployed along with their specialist kit to incidents at speed.