Police chiefs in England and Wales will take legal action in the high court against the government unless it backs down on plans to deduct hundreds of millions of pounds from their budgets, the Guardian has learned.

The unprecedented decision was taken by the National Police Chiefs Council, which sent a formal letter to the Treasury saying it will seek a judicial review of the government’s proposals.

It marks a new low in relations between police chiefs and the government. The NPCC represents police chiefs of the 43 local forces in England and Wales, and it has already hired a barrister to seek a judicial order forcing the government to climb down.

The dispute centres on the Treasury’s recalculation of the money each force needed to pay into the police pension scheme. This landed forces with a £420m bill, on top of the 19% cut in police funding since 2010.

Police chiefs say the reductions have left forces struggling to protect the public. Leaders of three of the largest forces estimate the move will leave them with officer levels last seen in the 1970s.

An NPCC spokesman said: “We have raised concerns about proposed pension changes affecting police force contributions with the Treasury and informed them of our intention to legally challenge the proposal if it goes ahead.”

One chief constable said: “The letter has gone to the Treasury and the chiefs are very perplexed as to why they would want to take this money off us.”

The budget further angered some in local forces that received no more money despite doing the bulk of policing.

The decision of the chancellor, Philip Hammond, to freeze funding came despite recent reports from the National Audit Office and the home affairs select committee saying cuts had left forces teetering on the edge.

The Police Federation criticised the chancellor’s award of £420m to deal with potholes, roughly the same amount he had declined to give police forces to solve their extra pensions payments. Hammond did give an extra £160m for counter-terrorism policing.

The Labour MP Louise Haigh, the shadow policing minister, said: “The government’s relationship with the police lies in tatters and it is unprecedented that police chiefs are now being forced to take legal action against ministers.

“Forcing the police at the last minute to bear the huge cost of pension changes demonstrates the utter failure of ministers to grasp the crisis in policing caused by their cuts.

“They have played fast and loose with public safety and the police are right to step up and take action.”

The NPCC has estimated the extra £420m bill, if met solely by cuts to police numbers, would mean 10,000 fewer officers on Britain’s streets.

Last week, three of the UK’s most senior chief constables – representing the West Midlands, Greater Manchester and Merseyside – warned of a fresh crisis in policing because of the pensions decision, which they said would leave them with no alternative but to cut officer numbers.

Dave Thompson, the chief constable of West Midlands police, said: “We are struggling to deliver a service to the public. I think criminals are well aware now how stretched we are. These further cuts will leave us smaller than we have ever been. There is unquestionably more demand than there was in 1974.”

The Merseyside chief constable, Andy Cooke, said the extra pensions bill would be crippling for his force. “The impact of the proposed changes on police officer pensions cannot, and should not, be underestimated. It is incumbent on me to ensure that those who will make the final decisions in relation to the pension changes understand the crippling impact these changes will have on policing,” he said.

The home secretary, Sajid Javid, has been trying to repair relations with the police since taking office. He said he believed forces should get more money, a reversal of the position of his predecessors Amber Rudd and Theresa May.

Javid is due to address police chiefs at their annual conference this week.

Last week, the Home Office said: “We are working closely with forces to understand the impact this change will have and are in discussions with police leaders about mitigating the impact on the frontline.

“The government is committed to continuing to ensure that the police have the resources they need to do their vital work, and the home secretary has been clear that he will prioritise police funding at the next spending review.”