Scans and blood tests sold by private health companies may be at best useless and at worst dangerous, says a leading specialist in preventive medicine.
Some experts say the radiation from CT scanners poses a risk
"Full body MOTs", which claim to spot early signs of disease, may often find benign abnormalities while missing real problems, says Professor Nicholas Wald.
At the same time, scanner radiation may actually increase cancer risk, he wrote in the Journal of Medical Screening.
The Department of Health is due to issue a report on the issue next week.
People can pay hundreds of pounds for private screening, which can involve a whole range of scans and tests to pick up potential problems.
They are designed to offer "peace of mind" to people who may not be suffering from any symptoms but who would like assurance that they have little to worry about.
But Professor Wald said in fact screening "always causes anxiety".
"Many abnormalities turn out to be false-positives frequently after sleepless nights waiting for the results of a definitive diagnostic test or procedure," he wrote.
"In medical screening, there is always some harm, which is only acceptable if there are also confirmed benefits that outweigh the harm."
The article singled out a brochure from Saga Insurance, which offered heart, colon, bone density, cholesterol and diabetes screening for £530.
The heart scan appeared to be a relatively poor tool for spotting problems, he said, while the benefits and harm of a "virtual" colonoscopy by CT scan had yet to be established.
Bone density scanning was a poor test for osteoporosic fractures, as was cholesterol testing for ischaemic heart disease, while screening for diabetes was still of "uncertain value".
Screening was being presented as desirable on the basis of a belief in its value, rather than on evidence of that, he added.
John Giles, an NHS consultant radiologist and clinical director of Lifescan, which provides the tests for Saga, rejected Professor Wald's assertions.
"We're not offering a full body MOT in any event. This is targeted screening which gives very clear results and is unlikely to cause unnecessary worry."
"You can't have a one size fits all policy: some people don't want to know, some people do. Screening is a personal decision and people are fed up with this paternalistic approach which tells them they can't make choices for themselves."
Next week, a panel commissioned by the government is due to report on the risks and benefits of CT scans.
It is due to conclude that screening healthy people is not recommended to deliver better public health.
"Screening should be carried out in controlled circumstances, and people need to be made fully aware of the benefits and the harm," says Gill Markham, vice-president of the Royal College of Radiologists, who helped draw up the report.
"People are being sent away with results they probably have no need to worry about, and then it's up to the NHS to pick up the pieces."