Hundreds more armed police officers are to be stationed across the country to deal with a Paris-style terror attack, the Prime Minister has announced.
However, the organisation that represents rank-and file officers questioned where the extra firearms personnel would come from.
Forces in England and Wales are starting to train around 1,500 extra armed officers as part of counter-terrorism efforts, with 400 ready to be deployed to cities outside London around the clock.
Che Donald, lead on firearms for the Police Federation Of England And Wales, said: "Any increase in armed police officers could be a good thing, but not if it depletes the frontline.
"Bearing in mind that police levels are at the lowest we've seen in decades, the question must be asked - where will these extra firearms officers come from?
"We know that they will have to come from existing strength, and with police numbers so low to draw from, we can't increase armed firearms officer levels without depleting the already thin front line and neighbourhood policing.
"Sadly, there's no armed police tree that we can pluck new officers from."
He called for a review into the legal protection for firearms officers when carrying or using guns, saying this was an issue in recruitment.
"The Prime Minister's announcement on firearm officer levels is good, but it needs more details than headline grabbing smoke and mirrors," Mr Donald said.
David Cameron made the announcement as he attended a summit in Washington, where world leaders are discussing plans to protect nuclear facilities.
He said: "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."
No 10 said the beefed-up network of armed police units is intended to complement military contingency plans already in place to deploy up to 10,000 troops in the event of a terror attack.
Soldiers would be drafted in to assist with the broader security response and relieve armed police guards who can, in turn, respond to incidents.
The recruitment drive aims to have 1,000 more armed police in place across England and Wales by spring 2018, with 400 ready to be deployed to cities including Manchester and Birmingham.
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.
Funding for the increase will come from the £143 million set aside in the Strategic Security and Defence Review.
Simon Chesterman, the National Police Chiefs' Council lead for Armed Policing, said: "This additional uplift will ensure we are in an even stronger position to respond quickly and effectively to protect the public."