The Department of Health has ordered an independent review into work done by Atos Origin for primary care trusts in the NHS North West region.
Atos has been working with eight PCTs since December but a much wider contract for the NHS North West region and the NHS South West region was due to start on Monday.
The£257m five-year contract was meant to provide 450,000 tests a year from 60 fixed sites and 17 mobile test centres.. But it has been delayed until the results of the independent review are released.
The move has left PCTs struggling to find alternative providers.
NHS North West said patients are being offered alternative appointments, either in the NHS or with another independent sector provider.
Atos was meant to provide CT and MRI scanning, ultrasounds, x-rays, audiology and other diagnostic tests to boost capacity and help PCTs hit next year's 18-week referral to treatment target. Atos would handle the entire process from appointment booking to test interpretation and report delivery.
The deal has been halted because ultrasound and MRI scan patients at PCTs in the North West already contracted with the company ahead of the main Department of Health-negotiated contract have had to be recalled for repeat tests.
NHS North West said: 'To date, there is no evidence that patient safety has been compromised.'
As part of the review, each scan or ultrasound is being re-examined and it is possible more patients will be recalled. No new patients are being referred to the interim service and the DoH has said the main contracts will only go ahead when it is certain patient safety will not be affected.
NHS North West chief executive Mike Farrar said: 'The problems we have had with Atos are primarily to do with their administrative procedures.'
In a statement, the company said: 'Atos Origin, in consultation with the Department of Health and the NHS, has taken the decision to stop accepting referrals for all diagnostic examinations while a full process review takes place.
'The three parties continue to work together in preparation for service delivery scheduled to commence in the coming months.'
However, the DoH said the start date of the main contract would have had to be delayed regardless of the problems in the North West.
'There are conditions in all our contracts which providers have to meet before we will allow services to commence,' said a spokesman.