Police are taking up to 30% longer than two years ago to react to 999 calls in parts of the country, with forces blaming deep spending cuts. Vital minutes have been added to the time it takes for a squad car to arrive at an accident or crime scene, an investigation can reveal.

Across the country, just under a third of police forces that collect response time information confirmed they reacted more slowly to emergency calls in 2012 than in 2010. More than half of forces had slower 999 response times in 2012 compared with the year before. Five forces said they had changed their target times since 2010, giving themselves more time to respond to 999 callouts. There was also a 23% increase between 2010 and 2012 in the number of calls not answered by the closest force and being handed on to a neighbouring one.

Such is the scale of the problem that in Bedfordshire, where the force changed its targets in 2011 from a 10-minute emergency response target to 15 minutes, a specific warning was made to staff not to complain publicly about cuts because of the risk of reducing confidence. The leaked email said: "I fully appreciate that we are all concerned with diminishing resources [sic] and external pressures, however by adjusting this behaviour we will quickly improve on this aspect of victim satisfaction."The new figures were obtained by the Bureau of Investigative Journalism during a three-month investigation, bringing to light a story of reduced police performance.

On coming to power, the coalition ordered police forces to cut budgets by 20% between 2011 and 2015. The number of staff has been reduced by more than 10,000 since 2010, according to government figures, with fewer police officers today than at any time in the last decade. Police forces had 500 fewer emergency response vehicles at their disposal in 2012 compared with 2010, according to the authorities that responded to freedom of information requests. The recent spending review for 2015-16 announced a further cut in central government funding in real terms of 4.9%.

Yet in recent weeks both David Cameron and his chancellor, George Osborne, have boasted that crime has fallen in spite of the cuts. Osborne told the Commons: "What was the opposition's prediction? They said that crime would rise. And what has happened instead? Crime has fallen by more than 10%." The figures revealed today, however, illustrate a decline in performance at times of emergency. Information obtained by freedom of information requests shows 16 of the 28 forces that record their response times displayed an increase in the time it took to arrive at a 999 emergency between 2011 and 2012. Of the 25 forces with figures going further back, eight said their response time had slowed between 2010 and 2012. Average response times for police forces in England and Wales also increased between 2011 and 2012, although only marginally compared with those with the most extreme plunge in performance.

Shadow home secretary Yvette Cooper said the revelations were "very worrying". She added: "These figures show the service the police provide to the public is being hollowed out. The police are doing what they can, but the scale and pace of the government's cuts over the last three years is hitting services despite Theresa May's promises that the frontline would not be hit. People want to know the police will be there when they need them. Because of this government's economic failure the problem is likely to get worse, with fewer officers travelling further to incidents."

Peter Neyroud, from the centre for criminology, University of Oxford, who specialises in policing, said: "While some of these forces have just increased by fractions of a minute these are annual averages and could be masking some serious lapses. The top responsibility of the police is to save lives, and so they ought to be prioritising that and ensuring they have sufficient units to cover getting to emergency response callouts."

At one of the forces with the worst record, assistant chief constable Robin Merrett of Sussex police, which has seen three minutes added to its 2010 average response time of 10 minutes, admitted performances had declined amid a "challenging programme of modernisation and savings". He said: "Sussex police acknowledges that since 2010 there has been a decline in the numbers of emergency and priority calls that are responded to within 15 minutes. This period has seen a 10% increase in the number of calls that needed a grade 1 emergency response. At the same time the force is delivering a challenging programme of modernisation and savings.

"We are aiming to put in place changes that will save money without the quality of services suffering. Many aspects of our service have been maintained and some have even improved. This is obviously not the case in respect of response calls and we will be looking at ways that we can improve our response to emergency and priority calls."

A Home Office spokesman said: "Police reform is working and recorded crime is down by more than 10% under this government. Like all parts of the public sector, the police must play their part in helping to tackle the deficit."

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