Tony Blair has promised not to "force through" plans to cut the number of police forces from 43 to 12.
His pledge came as police leaders from England and Wales prepare to lobby MPs to highlight opposition to the plans.
The Association of Police Authorities (APA) says rather than merge, forces should work together on issues such as fighting terrorism and organised crime.
Mr Blair argued that some forces might not have to merge and promised to listen to opposing views on the issue.
However, he rejected Conservative leader David Cameron's claim that his view was at odds with Home Secretary Charles Clarke's, adding: "We've got to listen to what people are saying."
He said the proposals had been prompted by the Association of Chief Police Officers which had argued that the current configuration of forces "was not satisfactory in fighting crime".
They also follow a report by Her Majesty's Inspectorate of Constabulary (HMIC), which said forces with fewer than 4,000 officers were not equipped to fight sophisticated modern crime.
"It isn't a question of forcing it through - it's a question of answering the point made by the inspectorate of constabulary," said Mr Blair.
Mr Cameron said the "real police reform agenda" was changing working practices, cutting paperwork and making them properly accountable.
Mr Blair said the government was already doing this but the problem was not "incompetent or lazy" police officers.
The exchanges came as Mr Clarke published the Police and Justice Bill, which aims to make neighbourhood police teams more responsive to the needs of local communities.
It includes plans to bring in a Police Improvement Agency to raise the quality of policing, improve local accountability, unify powers held by community support officers and help police fight anti-social behaviour.
Earlier, Mr Clarke told BBC Radio 4's Today programme that chief constables had "very strong views" in favour of force merger.
He conceded that "if you go region by region, you have got a range of different views held by chief constables and police authorities".
But he insisted: "The point is that the majority of police in Britain think this change is right."
However, APA chairman Bob Jones said merger costs could reach £1bn.
'Divide and rule'
Before Christmas, the police authorities ignored a 23 December deadline set by the home secretary to submit detailed plans for police force mergers.
The APA said 13 forces had expressed a desire to merge, 14 wanted to "stand alone", and the remaining 16 had no preference or were undecided.
The organisation has accused Mr Clarke of trying to "divide and rule" individual forces.
It argues that many prefer a "federated" approach, where neighbouring constabularies work together to deal with cross-border crime, serious incidents and roads policing.
However, Mr Clarke does not believe this would work, given the current command structure within the police service.
He insists that the development of larger "strategic" forces will help police combat terrorism and organised crime.
He also says the bigger forces will enhance neighbourhood policing.
Discussions about the mergers are continuing, and next month Mr Clarke may announce the first forces which have signed up to it.
Lynne Featherstone, the Liberal Democrat home affairs spokeswoman, urged Mr Clarke to look at alternatives to the merger plans.
"People will be expected to fork out more council tax to pay for the cost of the mergers, but at the same time they will have less of a say in how their police force is run," she said.