British Airways, which suffered bad publicity after a Channel 4 documentary revealed pilots drinking heavily the night before a flight, is one of a number of airlines in favour of random tests.
But such tests will "fail to protect passengers" and would have "serious flaws", says the British Air Line Pilots' Association (Balpa).
Captain Rick Brennan, a BA pilot for 32 years and chairman of Balpa, said: "Making the decision to report a colleague is tough..
"But if you're a real friend, you'll do it. If airlines co-operate we can deal with what are still very rare cases of pilots who misuse drink or drugs."
Drinking revealed
A Channel 4 TV documentary last October revealed drinking among a BA crew before a flight.
Last month the government announced a limit of 20 milligrams of alcohol per 100 millilitres of blood for aircraft crew as well as for air traffic controllers and aircraft maintenance engineers.
But the Balpa report said random testing had serious flaws.
Drinkers could continue to consume alcohol undetected. Heavy drinkers could excrete alcohol at a rate that would almost certainly allow them to test negative on the most stringent breath tests.
Balpa said a "peer intervention programme" has been running in the United States alongside random testing which was introduced 10 years ago.
Return to work
During that time, among a pilot community of 80,000, random testing had revealed only 80 instances of pilots being over drink limits.
Yet with peer pressure 550 pilots have been named, helped and returned to work.
It said if random testing was introduced by any UK airline, there must be safeguards against errors being made.
Drink problem
The report concluded: "A programme based on a punitive detection approach would drive behaviour underground."
Capt Brennan said: "As far as the public is concerned, even one pilot with a drink problem is one too many.
"Pilots with long-term drink difficulties will not be caught by random tests.
"Colleagues could be reluctant to report pilots with problems, but they would do so if they knew that pilots would get help."
Dr John McGurk, head of research and legal affairs at Balpa, said: "A peer pressure programme is certainly not about sweeping drug and alcohol problems under the carpet."