Information obtained under the Freedom of Information Act showed the number of officers classed as so-called "first responders" fell dramatically between March 2010 and December last year.
Labour described the decline as "shocking" while the Police Federation, which represents rank-and-file officers, warned the cuts could affect forces' ability to contain incidents such as last summer's riots.
It comes just weeks after official figures showed the total number of police officers in England and Wales had slumped to a 10-year low, with just 135,838 officers, following a four per cent cut in 12 months.
Ministers have claimed more officers are being switched to frontline jobs as police bureaucracy is reduced.
But the new data, supplied to Labour and verified independently by the House of Commons library, indicated a fall of 5,261 in the number of officers defined by Her Majesty's Inspectorate of Constabulary as being available to respond to 999 calls, as well as other crimes and emergencies.
Devon and Cornwall police lost a quarter of its first responders, 540 officers, while the West Midlands saw a fall of 19 per cent, or 1,023.
Paul McKeever, chairman of the Police Federation, said: "Those involved in responding are stretched and it puts their safety on the line. It also impinges on the public's safety."
Yvette Cooper, shadow home secretary, said: "To lose thousands of the very officers that you need in an emergency will be deeply worrying for people right across the country.
"Time and again the government has promised us the frontline will not be cut but now we see very clear proof that the very officers that need to respond to 999 calls, that need to respond to emergency incidents, are disappearing."
A Home Office spokesman said: "These are not official figures and we don't recognise them. The reality is independent reports have shown police can reduce costs while protecting the frontline and, according to official statistics and police plans, the proportion of officers on the frontline is rising."