Parents of 1,013 infants given single jabs between June and December last year are being contacted by NHS authorities to warn them that an incorrect process used for preparing the vaccines may also have put their young children at greater risk of other infections.

They are now being advised to have their children inoculated with the MMR free of charge after all, a recommendation that might only inflame the row over the NHS's refusal to offer the single jab in the first place. Parents worried by concerns that the combined MMR jab might have played a role in the rise in autism cases in this country or be a factor in Crohn's disease have been going private.

Lifeline Care Ltd, which runs the clinics involved, at Elstree aero-medical centre, Hertfordshire, and Hillsborough sports centre, Sheffield, has been reported to the National Care Standards Commission, responsible for ensuring standards in the private sector and investigating complaints.

It is understood David Pugh, the company's director, is being investigated by the General Medical Council, the profession's disciplinary body. This could provoke anger among parents who want single jabs, since there is a perception that doctors offering the private service are being picked on by the medical establishment.

Jackie Fletcher, of Jabs, the organisation representing families who believe their babies' health has been damaged by the standard MMR treatment, said she was concerned by allegations surrounding the Lifeline clinics.

But she added: "The (government's) medicines control agency are the overall body looking after this. If there are concerns about rules and regulations, they should be making single vaccines available at NHS clinics."

Dr Pugh seemed to be acting appropriately in offering parents blood tests to see if children were properly protected, she said.

An investigation began after concerns were raised with the Hertsmere Primary Care Trust last month in a confidential letter from two doctors who used to work with Lifeline Care.

Joel Bonnet, director of public health with the trust, said: "As a result of the changes in the way the vaccines were made up, there is a possibility that the efficacy of the vaccine has been effected, so that children are not as protected as normally they would be. There is a potential risk, which is why we are recommending parents get the children re-vaccinated with the MMR."