European laws tightening the emission limits medical staff can be exposed to from scanners will stymie research and harm patients, warn UK experts.
MRI scans are used to diagnose cancer among other conditions
By 2008, the Physical Agents Directive will put limits on exposure of operating staff to electromagnetic fields from zero to 300GHz.
But the limit is 100 times lower than that deemed safe for patients having the magnetic resonance imaging scans.
UK medics say the legislation is overly cautious and needs urgent revision.
They have written to the Secretary of State for Health urging the government to step in.
The government's Health and Safety Executive said it had already begun a dialogue with many sectors, including the medical industry, to look at how the directive may be implemented.
Experts are concerned the new laws will mean pioneering advances will be banned and patients, particularly in vulnerable groups such as children and those with cancer, will miss out on best treatment.
MRI scanners use magnetic fields to build up a picture of the inside of the body. The stronger the magnetic resonance, the more accurate the image.
Experts have always been aware that there is the potential for side effects and risks with their use.
There are theoretical concerns that exposure can cause biological changes in the body.
At very high doses, it can cause the muscles to twitch and some have reported feeling dizzy or sick when moving their heads quickly around a high field magnet, similar to the experience of a playground ride.
But in the few decades since their invention, no obvious health risks among operators or patients have been noted. About one million MRI scans are performed each year in the UK.
No good evidence
Currently, doctors weigh up the relative risks and benefits when deciding whether to use MRI.
MRI pioneers, including Professor Ian Young, senior research fellow at Imperial College London, and Nottingham University's Professor Sir Peter Mansfield, who won a Nobel Prize for his MRI work, say restrictions will ultimately put patients at even greater risk.
Research into interventional MRI - treating conditions with less invasive surgery guided by MRI, for example - would become illegal, they told a meeting of the charity Sense About Science.
Doctors would have to rely more, instead of less, on X-ray examinations which carry known, unwanted side effects.
Around half of MRI scans that are currently carried out on children with mild sedation would have to be done instead under general anaesthetic because parents and nurses would not be allowed to accompany and comfort children during scans, they said.
Patients will suffer
Professor Mansfield said the legislation was based on "very uncertain science" and warned it could lead to a medical brain drain, with experts moving to work in the US where the rules are not so strict.
For example, the Food and Drugs Administration sets an MRI exposure safety limit for patients that is 1,000 times higher than the level proposed for MRI operators by the EU.
Professor Young said this made no sense. "The EU claims there is an effect which is dangerous for fit staff that is not dangerous at 1,000 times the level for unfit patients."
The rules would also apply to other occupations that involve exposure to electromagnetic fields, such as train and tube drivers and the broadcasting industry.
Medical experts said the government should delay drafting UK legislation by two years and look into the issue related to MRI further.
Professor Stephen Keevil, of King's College and Guy's and St Thomas' Hospitals, London, said: "An exemption in terms of medical applications that look at the benefits weighed against the potential risks would be sensible."
Michael Clark from the Health Protection Agency said: "The medical professionals are right about the fact that there is a lack of evidence for deleterious effects.
"But we are dealing with new technology here.
"The Health and Safety Executive will have to implement this directive and they will need to consider such representations carefully."
The HSE said: "We are trying to set up a roundtable discussion where all interested parties come together to work out how to go forward with implementing the directive in a proportionate and appropriate manner."