The UK government did not do its own analysis of the cost of the biggest reorganisation of councils in England for decades, the BBC has learned.
Deputy Prime Minister Angela Rayner said "a significant amount of money" could be saved by merging councils in 21 areas into single authorities.
Rayner's department, the ministry of local government, based its cost estimates on a 2020 report commissioned by the County Council Network (CCN) that said £2.9bn could be saved over five years.
But the CCN has since revised its analysis and now says the reorganisation could make no savings and actually cost money in some scenarios.
The government will decide which proposals to take forward and intends to inform most councils next year.
A government spokesperson insisted the reorganisation "will improve services and save taxpayers' money".
Speaking to MPs in June, Rayner said: "Local government reorganisation will lead to better outcomes for residents and save a significant amount of money that can be reinvested in public services and improve accountability."
But her department insisted it was not necessary to commission separate in-house analysis of the cost of reorganisation at the public's expense, in response to the BBC's freedom of information request.
A PwC report in 2020 estimated potential savings of £2.9bn over five years if all councils in two-tier areas were replaced by single authorities.
But in
updated analysis this year, external, the CCN said the reorganisation could cost £850m over five years
and deliver no savings if 58 new councils, based on a minimum population of 300,000, were created in all 21 two-tier areas.
"Under this scenario no long-term efficiency savings would be delivered, meaning it would be more efficient to retain the current two-tier system in England," the CCN's analysis said.
Oliver said the CCN's analysis showed "that splitting county areas into unitary councils with populations as small as 300,000 will create hundreds of millions of new unsustainable costs for local taxpayers".
The District Councils' Network (DCN) said the reorganisation could produce poor results given the funding pressures already facing local government.
"It's astonishing that the government has undertaken no independent analysis before embarking on the biggest reorganisation of councils for 50 years," said Sam Chapman-Allen, chair of the District Councils' Network.
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