The first legal challenge to the government's controversial plans to merge some of the 43 police forces in England and Wales has been launched.
The government argues larger forces are more effective
Cleveland Police Authority wants a judicial review of the plan to combine its force with Durham and Northumbria.
Other police authorities and councils are poised to bring legal proceedings.
The Home Office, which believes larger forces are more effective, said it was "premature" to seek a review as no decision has been taken by ministers.
Derbyshire, Nottinghamshire, Leicestershire, Lincolnshire and Northamptonshire
Cambridgeshire, Norfolk and Suffolk
Bedfordshire, Hertfordshire and Essex
Surrey and Sussex
Cumbria and Lancashire
Cheshire and Merseyside
Cleveland, Durham and Northumbria
Staffordshire, Warwickshire, West Mercia and West Midlands
Dyfed Powys, Gwent, North Wales and South Wales
The merger plans were proposed last year after a report concluded the existing structure was not fit to fight organised crime and terrorism.
Charles Clarke, who was home secretary at the time, decided to cut the number of forces from 43 to 24.
He insisted that officers would not be taken away from their local areas to deal with such incidents.
Current Home Secretary John Reid has shown a willingness to slow down the merger process.
A spokesman for the department said Mr Reid would not make a decision until he had "carefully considered all views".
"The home secretary is working closely with police forces and authorities on police force restructuring," he said.
But Cleveland Police Authority wants Mr Reid to postpone the plans for a year and pursue other options, including better cross-border co-operation between forces.
The authority argues that the process is "unfair, irrational and perverse".
The authority's chairman, Dave McLuckie, said he had no choice but to issue the legal challenge.
"We have tried very, very hard to meet with John Reid and his team since his inception. We haven't been able to do that.
"The fact is that he hasn't listened to what we have said."
Labour MP for Middlesbrough, Sir Stuart Bell, also urged Mr Reid to "think again".
"It is a political decision that eventually affects the entire country," he said.
"A political solution whereby John Reid looks for alternative solutions is a better course of action."
In response, the Home Office reiterated that the strategic structure of the police services was not fit for the purpose, and that "the status quo is not an option".
The Liberal Democrats labelled the merger plans "damaging and unpopular".
The party's home affairs spokesman Nick Clegg said the legal action was "yet another nail in the coffin of the government's ill-thought-out proposals".
Other police authorities opposed to mergers may take legal action as well.
West Mercia will decide in the next few days on a course of action and Essex will do so later next month.
There is also the possibility of a challenge by local councils in the West Midlands, who are concerned about the impact on council taxpayers.
Meanwhile, the National Assembly for Wales - where four forces could combine - has sought legal advice over the plans.