John Honour, of University College London, said that the Government should act now to set standards for drugs testing, to avoid injustices against innocent people whose samples test positive. Testing for use of drugs such as cannabis, heroin and cocaine is one of the fastest-growing commercial areas in the UK.
Dr Honour told The Independent: "There aren't any standards for positive tests or for how the tests should be done ... If you failed a pre-employment test you might not even hear - you'd just not get offered the job."
A growing number of organisations, including Shell, BP, the Prisons Service, banks and British Rail have recently introduced random testing, and a number of companies - including Shell - ask applicants for some jobs to undergo drugs tests.
Dr Honour, of the Department of Molecular Pathology, has 25 years' experience in testing urine samples for steroids. In 1994 he began examining the tests carried out by a Spanish laboratory on a sealed urine sample provided by Ms Modahl.
That had shown a high level of breakdown products - metabolites - from the male hormone testosterone. That implied that Ms Modahl had taken the drug artificially.
But Dr Honour showed that bacteria found in urine could react with normal metabolites to produce testosterone. His evidence was crucial in reversing the August 1994 decision to ban Ms Modahl. Her name was eventually cleared last March.
He argues that similar problems could lurk for would-be employees as the use of drugs testing expands. "It's known that if you've been eating a bagel with poppy seeds on, you can test positive for opiates - drugs like heroin," he said. "The point is, each athletics test costs pounds 100, whereas companies are only paying about pounds 5."
The problem is that there is no clearly agreed standard for the minimum levels of metabolites which indicate the use of particular drugs.
"On the whole, the methods used are less sophisticated than those used by the IAAF, and there is no official regulation of companies that offer the tests," he said.
Unilabs, a London testing company, said that it was very careful to prevent "false positive" results. "We have a two-step procedure, so that if we find something in our first assay tests, we use chromatography and gas spectrometry to look for metabolites," said Fred Rutherford, head of scientific services.
People who claim innocence in industrial tribunals on drugs charges usually dropped their defence when evidence from Unilabs was presented, he added.
But not everyone adopts stringent standards. Dr Honour suggested that some testing companies might be profiting from the lack of standards in the fast-growing business of drugs testing.
"Most of the people in this area make their money from testing for companies. They do thousands of tests - so they don't want the bad side to come out. They would prop each other up," he said.