Seven of the UK's most senior police figures have threatened the Government with legal action over the "potentially serious implications" of further police funding cuts.
Six police and crime commissioners have been joined by Stephen Greenhalgh, London's deputy mayor for policing and crime, in urging the Government to delay a decision on force budgets expected in this month's spending review.
In a letter to policing minister Mike Penning, the group said changes to the police funding formula would result in cuts that are "unfair, unjustified and deeply flawed".
Mr Greenhalgh has signed the letter alongside the police and crime commissioners of the Cumbria, Lancashire, Devon and Cornwall, Merseyside, North Yorkshire and Thames Valley forces.
The letter said: "The uncertainty and concern caused to the communities we serve by the review as it currently stands was entirely avoidable and wholly unacceptable.
"It is with much regret that we are therefore taking legal advice with a view to initiating a judicial review, should our concerns not be addressed."
Arguing that the Government had ignored its own consultation guidelines in drawing up reforms to the police funding formula, it said: "Regrettably, we feel the most recent consultation to be wholly inadequate.
"We have been given just three weeks to consider and comment on major revisions to the initial proposals. This is not in keeping with the Government's own guidelines on consultation, which were also ignored for the original consultation."
The letter highlighted the effects police cuts could have on its signatories' forces.
It said: "Lancashire Police's budget will reduce by nearly 14% or £25 million per year, resulting in the loss of almost all of its proactive crime fighting and crime prevention capacity by 2020."
The letter added that "the future viability of Cumbria Police will be in question if the 15.8% or £9.4 million per year reduction in their central funding allocation is imposed."
Richard Rhodes, Cumbria's police and crime commissioner, said: "We are still in the consultation stage of the police funding formula and it is important to highlight that the Government has not followed its own consultant requirements.
"I am very concerned about the potentially serious implications for Cumbria from changes to the funding formula and I will highlight these when I respond to the consultation at the end of the month."
Anthony Stansfeld, police and crime commissioner for Thames Valley, said Thames Valley Police would lose more than £5 million next year if the funding is not changed.
He said: "The police grant funding formula as it stands is badly flawed and deeply unfair, and the process of consultation has been wholly inadequate.
"If the funding formula remains unchanged it will have a severe adverse impact on Thames Valley with our annual allocation of police grant decreasing by at least 2.6% or £5.6 million next year.
"This cut will be on top of the already anticipated Treasury 'Spending Review' funding cuts that we believe will require us to make further annual savings of at least £50 million by 2019/20."
Mr Penning said the current model for allocating police funding is "complex, opaque and out of date".
He said: "Police reform is working and crime has fallen by more than a quarter since 2010, according to the independent Crime Survey for England and Wales.
"But if we want policing in this country to be the best it can be, then we must reform further, and that includes putting police on a long-term, sustainable footing."
Chancellor George Osborne has asked ministers in non-protected departments - such as the Home Office - to come up with reductions in their budgets of between 25% and 40% by 2019/20 ahead of the Comprehensive Spending Review (CSR) on November 25, when the Government's plans for the next four years will be set out.
Metropolitan Police commissioner Sir Bernard Hogan-Howe has previously said he is worried for the safety of London if the Chancellor announces cuts of £800 million or more over the next four years, while Lancashire's chief constable Steve Finnigan warned that expected budget reductions of £60 million will mean his force will "not be viable as we see it today" by 2020.
Police chiefs, including Sara Thornton, chair of the National Police Chiefs' Council, are giving evidence to the Commons Home Affairs Committee later today.
The National Police Chiefs' Council said the letter was a matter for individual PCCs to comment on.
Steve White, chairman of the Police Federation of England and Wales, said: "It is deeply worrying that we have got to this point and I hope the government sees sense before imposing another round of savage cuts to forces' budgets.
"If ministers don't listen to police and crime commissioners then you wonder who they will listen to. Imposing further cuts to police budgets would be a wrong-headed and foolhardy move.
"Home Office figures show that since 2010 we have already lost 17,000 officers and a similar amount of police staff - equivalent to ten entire police forces. If we are to be able to properly protect the public, it is vital we don't see any more."