One of Britain’s largest police forces is threatening to launch a high court action against the home secretary as analysis reveals that Theresa May is secretly planning a major switch in millions of police funding from city forces to county constabularies, the Guardian can disclose.

West Midlands police are considering mounting the legal challenge to the way in which May has conducted a consultation exercise on a new police funding formula being carried out without the Home Office disclosing how each force is likely to be affected.

Relations between the force and the home secretary became so strained over the issue that the police have been reduced to making an unusual Freedom of Information Act request to demand disclosure of the figures.

A leaked internal analysis by senior police finance officials seen by the Guardian modelling the impact of the home secretary’s proposed changes shows that the major urban forces, including the West Midlands, would face deep cuts in funding as a result.

The figures from the Police and Crime Commissioners Treasurers’ Society (PACCTS) indicate that the Metropolitan police face a 43% reduction in their Whitehall grant in 2015-16 as a result of the funding formula changes under one likely scenario. The other big city forces in England also face deep cuts in their Home Office grants for next year, including 25% for West Midlands and Merseyside, 23% in Greater Manchester and 18% in Northumbria.

Under the same scenario, the PACCTS said that leafy shire forces would receive increases, including 75% for Lincolnshire, 71% for Dorset, 68% for Warwickshire, 65% for Wiltshire, 60% for Gloucestershire, 59% for Suffolk and a 52% increase for Surrey. Eight of the 43 forces – all of them major city forces with Labour police and crime commissioners – would be losers as a result of the changes.

The consultation over how £7.8bn of police grants are distributed among the forces in 2015-16 is on top of a major Treasury exercise to find 25% to 40% cuts over the next four years in “unprotected” departments such as the Home Office.

The disclosure of the scale of withdrawal of millions of pounds from inner-city policing is likely to fuel the tensions between police and the home secretary.

Police force could lose 22,000 jobs under new spending cuts Read more

Earlier this week, the Guardian disclosed that senior officers were working on the assumption that a further 22,000 police jobs could disappear by 2020 as a result of the next round of spending cuts. The deadline for “unprotected” departments to submit their initial plans for reductions to the Treasury passed on Friday. The Metropolitan police commissioner, Sir Bernard Hogan-Howe, has told his officers that “the time has come to fight our corner” over police cuts. “Watch this space,” he added.

The Home Office responded earlier this week to the West Midlands’ FOI request by saying it would disclose the figures but not until 29 September – two weeks after the formal consultation ends on 15 September. The West Midlands force fears that the chancellor’s next round of spending cuts and the police funding formula changes could result in it losing 55% of its funding within five years.

The force’s chief constable, Chris Sims, told a West Midlands strategic police and crime board meeting this week that it was now considering a legal challenge. David Jamieson, the West Midlands police and crime commissioner, said the force was facing the most serious situation since its formation in 1974: “Any approach that protects the people of the West Midlands by giving us a fair deal won’t be ruled out. I am not one to cry wolf or say that cuts cannot be managed or efficiencies made, but the course that the government is taking us on is beyond that.”

Labour’s policing spokesman, Jack Dromey, said the leaked analysis from the senior police finance officials confirmed his worst fears: “The government’s approach over the past five years to police funding has been characterised by a grotesque unfairness. The West Midlands, for example, has been hit twice as hard as Surrey. Now instead of improving the situation, the government’s proposals are set to make it even worse. These revelations are extremely concerning and make it even more imperative that the government comes clean with the public about their secret plans.”

The policing minister, Mike Penning, said police funding had to be put on a long-term sustainable footing: “The current model for allocating police funding is complex, opaque and out of date. That is why we are consulting on plans to reform the allocation of central government funding to police forces in England and Wales, ensuring it is fair, robust and transparent.

“Final allocations for police forces have not been set and will not be determined until we have consulted on the principles.”