Chauffeur-driven luxury cars are being used to transfer prisoners between jails because it is cheaper than running armoured vans.
Luxury cars are used because they are "reliable"
Inmates from Holloway, Pentonville and Wormwood Scrubs in London are driven in £30,000 E-class Mercedes Benz cars which have leather seats and air conditioning.
On Tuesday a spokesman for the prison service said that it was cheaper to use contracted taxi firms rather than run vans, but stressed there was "no compromise on safety".
All the drivers are vetted by the Home Office.
"Holloway is not a high security prison," the spokesman said.
"Taxis are considerably cheaper than using prison vans which need maintaining.
"However, there is no compromise on safety. Prisoners are always risk assessed before leaving the prison."
The spokesman added that the choice of vehicles used was down to the car firm.
Defending the use of luxury cars, a spokesman for Corporate Cars, which is based in Whetstone, London, said they were used because they were reliable.
"Cars have been used before which are older and more unreliable. They break down. We are getting the prisoners to their destination safely and securely.
"What would the public say if we went the other way and used unreliable old cars, which could break down in traffic?
"What would happen if a car broke down in traffic with the prisoner sitting there?
"If a car breaks down anywhere, it is the security risk that is paramount and we need to use reliable cars to make sure this does not happen."