Security firm took inmates to court in taxis

Serco signed the seven-year deal in March. It is worth £42m per annum to them and there is an option for a further three years.

However the company, whose deal covers London and the east of England, has recently been using scores of private taxis after a computer system organising pick-up and drop-off details failed.

The Ministry of Justice confirmed that taxis were being used.

Conservative MP Patrick Mercer said he was going to raise the issue with Home Secretary Theresa May.

Mr Mercer, a former soldier, said he was worried about the cost, the perception defendants were being driven round in luxury and also by the security concerns.

He said: “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.

“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 taxis were only used to move juveniles and women who had been assessed and classed as compliant, non-violent and of a low escape risk.

Such prisoners are double handcuffed, their hands bound together and also attached to one of two guards.

Prisons will not accept inmates back after a certain time in the evening and Serco incurs financial penalties if defendants are locked out for the night.

It costs around £55 to lock up a prisoner in a vacant police cell.

The business said it had used about 80 black cabs in around 25,000 journeys since August because they were more reliable than relying on custody vans that might not get defendants back on time.

A Serco spokesman said: “Serco is committed to providing a high quality prisoner escorting and custody service in London and the east of England.

“For the last few weeks we have experienced some transitional problems with the systems we use to manage the hundreds of vehicle movements we conduct daily, and this has sometimes affected the service.

“On a number of occasions, and for a variety of reasons, it has not been possible to return adult prisoners to prisons on time after court appearances and they have been accommodated in police stations.

“When this is Serco’s responsibility, we incur a financial penalty.

“To optimise capacity for the transport of higher risk prisoners and to ensure that certain categories of vulnerable prisoners (such as juveniles and females) arrive back at their allocated young offenders’ institute or prisons as early in the evening as possible, in exceptional circumstances we have used an approved and pre-contracted taxi company to supplement our own fleet of specialist vehicles.

“We have only done this where the prisoner is compliant, is non-violent, and does not present an identified risk of escape.

“An escort of two Serco prisoner custody officers is always in attendance. Serco pays the additional costs incurred when prisoners are moved in this way.

“The security and safety of prisoners remains our top priority at all times.

“In co-operation with HM Courts and Tribunals Service, the National Offender Management Service and the police, we are working to resolve the current operational issues and restore the service to the standard of efficiency which we, our customers and the public expect.”