There could be "serious implications" for Gloucestershire Police, the police and crime commissioner has warned.
Cotswold District Council's proposal to form a unity authority with West Oxfordshire has proved controversial.
But CDC says the plans - dubbed 'Coxit' - are at an early stage but aim to improve accountability.
CDC leader Lynden Stowe has said Gloucestershire's "historic borders" would remain under the proposals, which aim to improve the "administration and the delivery of council services".
He told BBC Radio Gloucestershire a few weeks ago: "We would expect the police to patrol up to the existing county borders, exactly as now, so if there's a crime in Cirencester, Gloucestershire Police attend and if there's a burglary in Burford, Thames Valley Police attend."
But Gloucestershire's PCC Martin Surl said he had taken legal advice on the "Coxit" proposal, and warned it could "signal the end of Gloucestershire Police as we know it".
Under the Local Government and Public Involvement in Health Act 2007, he said the Cotswolds could become part of Thames Valley Police District.
"If they break away to a new district, we can't continue policing just half a district," he told the BBC.
"So, either we start taking some of Thames Valley's area in Oxfordshire, or they start policing parts of Gloucestershire. Neither want that ."
A spokesman for Cotswold District Council said they "fully understand the initial concerns" but they were "best addressed through the detailed studies that will now take place to assess the feasibility of the unitary proposal".
Mr Surl, an independent, is standing for re-election as Gloucestershire PCC on 5 May, alongside Labour's Barry Kirby and the Conservatives' Will Windsor Clive.

Carter has suffered with the condition for years and struggled throughout the 6-2 defeat by the three-time champion at the York Barbican.
Carter, 36, told BBC Sport: "I have such a handicap living with this Crohn's disease.
"When you have constant abdominal pain you can't settle."
Carter, who has recovered from a life-threatening cancerous tumour on his lung, scored a fluent 89 in the opening frame.
But Higgins lived up to his billing as a pre-tournament favourite, barely missing a ball thereafter.
He capitalised on a couple of Carter misses to pinch the next two frames and rattled off five frames in succession before closing out victory.
Two-time World Championship finalist and world number 28, Carter, added: "I missed a couple of balls and am 3-1 down.
"It is so hard to compete at the top level anyway and when you have something like that, it makes it very difficult.
"I don't want to keep going on about it and I don't feel like my game is a million miles away. But it is such a big handicap If you are not 100% right competing at this level."
"It's a wonder I can compete at all. It's not an excuse, it's a handicap. But in the bigger picture, I am lucky to be alive I suppose."
The Cotswolds could be policed by the Thames Valley force if plans to "break away" from Gloucestershire County Council go ahead, it is claimed.
[NEXT_CONCEPT]
Ali Carter said his battle with Crohn's disease put him at a huge disadvantage during his third-round UK Championship loss against John Higgins.