The Ministry of Justice has admitted a private security firm is using black cabs to take prison inmates to court.
The firm, Serco, has a seven-year deal - with a further three-year option - worth up to £420m.
But the company has been forced to transport inmates in London and east England in cabs after their computerised booking system failed.
Conservative MP Patrick Mercer said he would raise the issue with Home Secretary Theresa May.
Mr Mercer said he was worried about the cost of hiring cabs, and the perception defendants were being driven around in luxury.
"I just think it is ridiculous that a security company such as Serco misjudges things so badly that prisoners are moved to and fro in black taxis," he said.
"A taxi is, by definition, an extraordinarily expensive means of transport and the government will pick up the tab for that.
"If they are in a taxi with a warder handcuffed to them that's one thing, but it's going to be part of my request to the home secretary to find out exactly what the security arrangements are."
Serco said the taxis were only used to move juveniles and women who had already been assesed to be non-violent and a low escape risk. The prisoners are double-handcuffed and always accompanied by two prison officers, the company said.
Prisons are not allowed to accept inmates back after certain times in the evening, and Serco was charged £55 a time to lock up prisoners in vacant police cells. The company said some 80 black cabs had been used for 25,000 journeys since August.
A Serco statement said: "The security and safety of prisoners remains our top priority at all times."