The number of police forces in England and Wales could be reduced from 43 to 12 under plans announced by home secretary Charles Clarke.
Some senior officers want a single national force
The restructuring proposals were announced by Mr Clarke on Thursday as part of an ongoing review.
Advocates of the changes say larger police forces work better and the plans could save up to £2.3bn over 10 years.
But the Tories and Lib Dems argue the reorganisation will break the link between police and local communities.
'Only a steer'
Mr Clarke suggested that in Wales he would only look at merging the current four forces into one.
And in the North East, he appears to be backing the idea of combining the Northumbria, Durham and Cleveland constabularies into one force.
Conservatives Stephen Dorrell and Peter Lilley say it was wrong for chief constables to lobby their local MPs ahead of a Commons vote on the issue
The plans follow a report in September from Her Majesty's Inspectorate of Constabulary saying the current force structure was "no longer fit for purpose".
The changes would be seen as the biggest overhaul in policing for more than 30 years.
A Home Office spokeswoman said: "The number of forces could be as few as 12 but only if the forces choose to take that option."
She added: "This is just a steer from us. They do not have to follow our guidance. The home secretary is simply saying: 'This is my preferred option for you.'"
Shadow home secretary David Davis said the Conservatives would fight the plans.
"Replacing the current structure with regional police forces will seriously damage the relationship between local areas and their police," he said.
"As the bonds between communities and their police forces disappear, local accountability will be eroded."
Lib Dem home affairs spokesman Mark Oaten echoed those concerns.
He said: "Efficiency savings could be made by sharing resources between the smaller forces and by setting up a national resource to help forces deal with particularly complex investigations.
"It seems that once again the government is trying to impose more central control over policing, instead of giving greater power to local communities."
Mr Clarke has written to chief constables proposing the following options for each region:
East Midlands - Either one East Midlands force; or two forces - one covering Derbyshire and Nottinghamshire, and another for Leicestershire, Lincolnshire and Northamptonshire
Eastern - Either one Eastern force; or two forces in one of two different combinations: Bedfordshire, Hertfordshire and Essex plus Suffolk, Norfolk and Cambridgeshire
OR Norfolk, Suffolk and Essex plus Hertfordshire, Bedfordshire and Cambridgeshire
London - No change proposed - reviews are continuing into the future of the City of London police and British Transport Police in the city
North East - Create one force for the region by merging Northumbria, Durham and Cleveland
North West - Either two forces - one for Lancashire, Cumbria and Merseyside and another for Cheshire and Greater Manchester Police
OR three forces - one for Lancashire and Cumbria, another for Cheshire and Merseyside, and a separate Greater Manchester force
South East - Any of the following options:
Two strategic forces - Kent, Surrey and Sussex plus Thames Valley and Hampshire
Three forces - one for Kent alone, another for Thames Valley and a third for Surrey, Sussex and Hampshire.
Three strategic forces - Kent, Surrey and Sussex; Thames Valley as a standalone force; plus Hampshire as a standalone force
Three strategic forces - Kent and Sussex; Thames Valley as a standalone force; plus Hampshire and Surrey
Four strategic forces - Kent as a standalone strategic force; Thames Valley as a standalone strategic force; Surrey and Sussex; and Hampshire as a standalone strategic force
South West - Either one regional force
OR two forces comprising Devon and Cornwall as a standalone force, alongside one covering Avon and Somerset, Gloucestershire, Wiltshire and Dorset
Wales - A national Welsh force merging North Wales, Dyfed-Powys, Gwent and South Wales
West Midlands - Either one West Midlands force
OR two forces - Staffordshire and West Mercia; plus Warwickshire and West Midlands
Yorkshire and Humberside - Either a single regional force
OR two strategic forces - one for West Yorkshire and North Yorkshire; another South Yorkshire and Humberside.