It is part of the government's drive to cut waiting times for examinations to diagnose conditions such as cancer.

But doctors have criticised the five-year deal with Alliance Medical, which began in the summer, saying it has resulted in mistakes and delays.

The private company checks NHS patients in mobile scanners and sends the images to be read abroad.

Delays and mistakes

This is because they are not allowed to use NHS staff to interpret the images.

But senior hospital managers have written to the Department of Health to complain that the contract was hurried through, and failed to specify proper standards, resulting in sub-optimal care.

They said there had been cases where the results had been delayed, or the images misinterpreted, by radiologists in countries such as Belgium, Spain and South Africa.

It's hugely beneficial to patients Health Secretary John Reid

Dr Anton Joseph, a consultant radiologist at Mayday University Hospital in Croydon, said: "Fortunately, in the instances that I know of, no harm befell the patient because the clinician recognised that the diagnosis was likely to be wrong."

Dr Gill Markham, chairwoman of the British Medical Association's radiology sub-committee, branded the contract "a complete disaster".

She said the money could have been used to boost existing NHS scanners which can sit unused for half of the week due to a shortage of staff and resources to run them.

Alliance Medical acknowledged that there had been some delays and mistakes but said these problems were being addressed.

Teething problems

Managing director John Walsh said: "One or two [hospitals] did express concerns about slow turn around of reports.

"We are going through quality assurance programmes with them."

John Reid said: "There have been teething problems."

But he said the standard of accuracy and reliability of the service was as good as that seen in the NHS and it was leading to big falls in waiting times for patients.

In Huddersfield, waits have gone down from 38 weeks to eight weeks and in Scarborough patients are now waiting only five weeks for non-complex scans compared to 30 weeks previously, he said.

"From the patient's point of view, the monopoly of the NHS has meant waiting lists for years that are now plummeting.

"And because we are doing this, we will have scans in three weeks in the NHS and an 18 week end-to-end waiting list.

"It's hugely beneficial to patients."

The government's target is a maximum 18-week wait between seeing a GP and NHS hospital treatment in England by 2008.

It has pledged to pay private health firms �1bn to carry out hundreds of thousands of diagnostic tests on NHS patients to this end.

Dave Prentis, general secretary of Unison, said: "The medical profession have raised grave doubts...and these must be taken seriously and dealt with quickly."

He said government should concentrate on building up capacity in the NHS.