Security guards in Darlington say they are filling a vacuum left by the police
The growing number of private security companies policing UK streets is a worrying development, senior police figures say.
The Police Federation of England and Wales said there is "huge concern" over their powers and accountability.
Former Metropolitan Police Commissioner Sir Ian Blair has also said there should be no role for the private sector in Britain's law enforcement.
The firms typically charge residents to patrol streets and deter troublemakers.
The growth in private security firms taking on policing work comes despite an increase in police numbers.
A record 141,252 police officers are available for duty in England and Wales, although there have been reductions in 16 police areas.
Private security firms have no powers, although chief constables may award some limited ones such as allowing them to move people on.
BBC correspondent Keith Doyle joined one private security company who began patrolling the streets of Darlington this week.
He said residents there pay between £2 and £4 a week to have their homes included in regular patrols and to receive an instant response if they need help.
"These guards know they have no powers but they say simply by being here it prevents trouble and that's something local residents agree with and have signed up for," he said.
Francis Jones of Sparta Security told our correspondent the patrols provided a visual deterrent to potential criminals.
He said: "We are giving a deterrent to them and also raising the confidence of the public who have taken us on board and there are quite a lot of people coming forward, ringing us up, wanting our service."
'Fear of crime'
But the vice chairman of the Police Federation, which represents officers, said such firms could cause problems.
HAVE YOUR SAY
If the police were actually seen on the streets there would be no need for these private security firms
Gordon W, Manchester
Simon Reed said: "We have got people who have certain powers, we are going to see them in uniform. Potentially there is confusion there for the public and who are they actually accountable to?
"I understand the public's fear of crime but actually it's the police who patrol public space and we should be very wary about giving those powers to private security companies."
Sir Ian Blair said more use should be made of community officers and civilians working within the police, otherwise there could be more private police patrols.
He said: "I do not see community safety as a commodity to be bought and sold and therefore we shouldn't be having the private sector in policing.
"Unless we get this right, we will end up with private security coming in and they will work for the rich and the poor will go without."