Metropolitan Police officers are to start using spit hoods in a pilot scheme next month, despite claims the measure is "primitive, cruel and degrading".
The restraint device is intended for use on unruly prisoners to prevent them from biting or spitting on officers.
It will be initially used in 32 police station detention areas but not on London streets or in public.
The hoods, which are classed as a use of force, hit the headlines when officers from British Transport Police were filmed using one on a man at London Bridge station in July.
Martha Spurrier, director of human rights group Liberty said: “A spit hood is a primitive, cruel and degrading tool that inspires fear and anguish.
"We have seen many cases where the police use them unnecessarily and without justification, including on children and disabled people."
“Police have the power to use force against citizens when they have to – using handcuffs, arm restraints, leg restraints, pepper spray, batons.
"The suggestion that officers need to be able to cover people’s faces and heads is as far-fetched as it is frightening.
"Spit hoods belong in horror stories, not on the streets of a civilised society – we urge the Met Police to think again.”
The Met has stated the hood is considered a "use of force" and that officers were being trained to use it only when necessary.
The Police Federation has called for the implementation of the spit hood to help protect its officer members.
A force spokeswoman said: "A pilot scheme has been approved for the use of spit guards in all Metropolitan Police custody units from October 2016.
"These guards will only be used in a custody situation. They are considered a use of force and so officers are being trained to ensure that their use is proportionate and necessary in the circumstances.
"The use of this protective equipment is necessary to meet the duty of care owed to officers when a detainee spits at or attempts to bite them."
