Police volunteers could be used in cases of terrorism and child sexual abuse, Labour's shadow home secretary Andy Burnham has warned.
Mr Burnham is to push for a Commons vote next week to commit the Government to protecting the police budget before new rules to recruit volunteers can be used.
Labour also want to halt Government plans to give volunteers the same powers as constables, and prevent unpaid officers from attending scenes where force or restraint may be required.
Mr Burnham said that ministers have failed to rule out using volunteers in sex-abuse and counter-terrorism operations in written parliamentary answers.
"There's a proud tradition of the public volunteering with the police, but not replacing them. Labour is calling time on this plan - the Government cannot cut the police budget and ask the general public to take the jobs.
"Just when security concerns are paramount, they have hatched a plan for thousands of volunteers to attend even the most serious cases. Public safety is at risk if the police cannot mount experienced officers in the event of an attack," Mr Burnham said.
Home Office Minister James Brokenshire said police chiefs need to have flexibility in deploying people.
"Our consultation on the reform of the roles and powers of civilians and volunteers demonstrated the demand from forces for flexibility in how they deploy volunteer staff, and therefore we should not make assumptions about the operational areas where volunteers can make a contribution. For example, they can play a hugely valuable role in supporting vulnerable victims.
"Volunteers will be subject to the same tests as police staff before they are designated with powers, namely that the chief officer must be satisfied that they are suitable, capable and adequately trained.
"If it is acceptable to confer all the powers of a constable on one type of volunteer, namely special constables, it is inconsistent to object in principle to conferring a narrower set of powers on other suitably trained volunteers," the minister said in a written parliamentary answer.