Political divisions over Government plans for directly-elected commissioners to run police forces will be exposed at a high-level conference tomorrow, a leading Westcountry councillor has warned.

David Cameron has vowed to press ahead with the reforms, which would see police authority boards replaced with elected commissioners, despite fears the powerful posts could be hijacked by right-wing extremists.

However, Councillor Brian Greenslade, a former chairman of Devon and Cornwall Police Authority and head of the Liberal Democrat group on the Association of Police Authorities (APA), warned the Prime Minister that opposition was strengthening, even among Tories.

Mr Greenslade said Mr Cameron was "wedded to the idea", whereas the "the Conservative group on the APA are as opposed to this as everyone else".

He told fellow members of the police authority that they had said they had tried to "tease out" more details about the proposals from Police Minister Nick Herbert last week but were only told that it was a "manifesto commitment".

Last week, Mr Cameron met with senior officers from across the country, including Devon and Cornwall Chief Constable Stephen Otter, to discuss the issue.

Mr Greenslade said the APA was "not best pleased that no similar invitation has been offered".

"A lot of opposition building up," he added. "I think police authorities are not broken and don't need fixing in this way."

He said a resolution would be made by the APA ahead of its joint annual conference with the Association of Chief Police Officers (ACPO), which starts in Manchester tomorrow morning.

The pre-election pledge by the Conservatives was to introduce directly-elected commissioners across the country – modelled on the current situation in London – to oversee the work of chief constables.

The Lib Dems committed to having directly-elected members onto police authorities. In Devon and Cornwall, the authority has 19 members – ten are elected councillors from the region's largest councils, with nine independent members, including two magistrates.

Mr Greenslade, who has an upcoming meeting with Deputy Prime Minister Nick Clegg, said Mr Cameron's plan would not work and that they could "provide evidence to show the police authority has worked successfully here".

The authority is well regarded nationally, particularly for its financial management. It also proved its effectiveness in forcing the retirement of former Chief Constable Maria Wallis in 2006.

Police Minister Nick Herbert has insisted having directly elected individuals in overseeing policing in the Westcountry will "help shed bureaucracy and make forces more accountable to the people they serve".

He has also stressed to chief officers that the new commissioners would not be able to interfere with "the power of chief constables to make operational decisions on how to run their forces".

Details on exactly how the new system would operate, including how and when elections for the posts would be held, remain sketchy.

Concerns have also been raised about the size of each commissioner's responsibility. If there was one for each large council area, the person covering Devon would represent more than 700,000 people, while the Isles of Scilly would have just 2,150.