The arrangement, where the forces have shared certain functions such as forensics and computer services, is due to come to an end on Tuesday.
West Mercia’s police and crime commissioner John Campion said the force was fully prepared for the break-up, but that the Warwickshire force had asked for a transition period.
However, Mr Campion’s suggestion that any such arrangement would be dependent on agreeing a satisfactory financial settlement between the two forces has been met with anger by Warwickshire Police Federation.
The Federation’s chairman, Pc Simon Payne, accused Mr Campion of using public safety as a bargaining chip in the negotiations.
Mr Campion denied the claims, and said he was surprised that neither the Warwickshire police and crime commissioner Philip Seccombe nor Chief Constable Martin Jelley attended talks due to have been held on Monday.
Pc Payne said: "John Campion seriously needs to reflect on his comments concerning the withdrawal of frontline facilities that are vital to the protection of the public in Warwickshire.
"The alliance was a ground-breaking collaboration that was recognised by the highest levels of government, saving the two forces tens of millions of pounds.
"It was the decision of West Mercia to terminate this agreement. There are going to be cost implications, and that resides with West Mercia Police."
Mr Campion defends his comments, saying his first responsibility was the safety of the public in West Mercia.
He says: “Public safety and policing in Warwickshire are matters for the county’s chief constable and police and crime commissioner.
"My responsibility relates to the safety of communities in West Mercia and ensuring they receive an effective, efficient police service."
About 80 per cent of shared services are currently based in the West Mercia force area, which covers Shropshire, Herefordshire and Worcestershire.
These services would need to be reinstated in Warwickshire after the split, however new terms are yet to be agreed on how to terminate the alliance.
"We won't let services suffer while we work through negotiations," says Warwickshire's chief constable Martin Jelley.
The split has also attracted criticism from HM Inspector of Constabulary, Wendy Williams, in an inspection report published last week. She says the decision "does not appear to have been based on a well-evidenced business case," adding that there is "no certainty how Warwickshire will provide services without the support of West Mercia."
West Mercia Police says the report findings are out of date.
Chief Constable Anthony Bangham says: "We are in a fundamentally different place from when the inspection was carried out," he says.
Mr Campion says the alliance, which was brokered by his predecessor Bill Longmore in 2012, is not working for West Mercia.
"There are numerous examples of where West Mercia is subsidising Warwickshire, to its own detriment, and that cannot continue," he says.
Mr Campion denies threatening to withdraw services from Warwickshire.
"In fact West Mercia has made multiple reasonable offers to continue collaboration beyond October," he says. "To date, Warwickshire has rejected all of these.”
Mr Campion has previously described the alliance 'unfair' and 'one-sided', arguing that Warwickshire Police took more out of the arrangement than it put in.
“For example, I have just spent £1.47m of taxpayers’ money refurbishing forensics at Hindlip, which is currently an alliance function,” Mr Campion says.
“Warwickshire will have contributed nothing to that because capital projects are not part of that alliance."
Pc Payne firmly disputes these claims, and calls on Mr Campion to retract them.
"I strongly object to the intervention by the West Mercia police and crime commissioner that West Mercia are effectively subsidising Warwickshire, and I would ask him to apologise to every police officer serving in Warwickshire," he says.
Mr Campion says Pc Payne is not aware of all the relevant facts, and says he is contradicting his own argument.
"He claims that West Mercia does not subsidise Warwickshire, while simultaneously claiming there could be risks to the public in Warwickshire if West Mercia stops collaborating," says Mr Campion. "One of those two claims cannot be accurate.”