The flagship scheme for certifying builders has been called into question after a string of test centres were caught rigging health and safety exams.
Construction workers across the UK are required to hold a CSCS card to prove skills and grasp of health and safety.
But a joint BBC London/Newsnight investigation revealed widespread, organised cheating, allowing untrained builders on to dangerous sites.
The scheme's administrators insist fraud is being tackled robustly.
Construction Skills Certification Scheme cards, launched by the industry in 1995, are seen as a benchmark.
Nine of the UK's 10 biggest construction companies demand them, as do Crossrail and Heathrow.
But numerous test centres are offering guaranteed passes for cash, enabling workers lacking English to obtain qualifications.
Wep HSE, in Ealing, west London, was among three centres exposed by undercover reporters.
Its director, Garet Estensen, read exam answers from a big screen, instructing candidates: "Follow me on screen, guys. I'm going to shout the correct answer, you just click.
"We're going to make a couple of mistakes - what I don't want is everyone making the same mistake."
Find out more
Watch an extended report on Newsnight at 22:30 BST on BBC2. Or catch up via the iPlayer.
Confronted with the evidence, Mr Estensen, who has won a prestigious health and safety award, made no comment.
Also exposed was Future Training 4 Jobs in Ilford, east London. For £450, Anna Calancia issued the BBC with a supervisor qualification.
The company says Ms Calancia acted alone. It insists it provides quality training and works hard to uphold standards.
A CSCS card acquired with fraudulent qualifications was used by the BBC to get offers of work at building sites, a school - and even a power station.
Construction is the UK's most dangerous employment sector. More construction workers were killed than members of the armed forces during the Afghanistan war.
In the past five years alone, 221 workers died.
The cost of untrained workers
Alan Gillman's son Justin, 26, was killed by falling bricks on a building site in Lincolnshire.
He and another worker were untrained for their task and Chestnut Homes was fined £40,000.
Mr Gillman said: "You don't expect someone to go to work and not come back. It's beyond all belief.
"He would've been married by now, we would have grandchildren.
"That's all been taken away. You think about it every day."
A BBC Freedom of Information request to the Construction Industry Training Board (CITB), which administers the scheme, revealed steeply rising cases of CSCS fraud.
There were 96 reports in 2012, some 264 in 2013 - and 311 last year.
Construction company Willmott Dixon, which has a turnover of more than £1bn, has now raised questions over the entire scheme.
Mark French, head of safety, said: "As an industry, we've set our stall on the CSCS card being the minimum benchmark to accept workers on to our sites.
"If these people aren't competent workers, we'll end up with guys prepared to take risks. It's going to take a long time for us to get over this as an industry. We'll probably never identify the true number working with cards that aren't bona fide."
Health, safety and environmental manager Alistair Donaghey warned of possible deaths, adding: "I'll never trust the scheme again."
It is estimated that 14% of the UK's construction workers are from abroad - approximately 228,000 people.
Reports by Loughborough University and Irwin Mitchell solicitors have warned that communication difficulties put them at higher risk of accidents.
T&D Glazing and Installation, in east London, employs 200 people. Managing director Toni Timis estimates 20% applying to work there have fraudulent CSCS cards.
He said: "Out of 100 people, half will speak English, so they have no problem with exams. Of the other half, 15 or 20 pay someone [extra] rather than take exams. It's easy to spot them."
One Romanian labourer, speaking anonymously, told the BBC it was open knowledge that CSCS qualifications could be bought.
He said: "People don't care about it being illegal - they just get a job, make some money. In the future, they'll kill someone."
A CITB survey of 419 construction workers responsible for checking CSCS cards indicated 1% saw a fraudulent card every day.
In London, one-third had seen a fraudulent card.
Carl Rhymer, of the CITB, said: "CITB is aware of the problem card fraud poses, which is why we've taken a series of measures to tackle this head on.
"We doubled our spend on fraud investigations, which led to five centres being shut down - with eight other centres under investigation.
"We're accelerating plans to install mandatory CCTV in all centres to monitor for fraudulent activity, and have launched spot-checks.
"Our intelligence suggests card fraud is focused in a small minority of the 544 testing centres."
Graham Wren, the scheme's chief executive, said: "CSCS takes fraudulent activity extremely seriously, and it's essential that steps are taken to prevent it.
"CSCS relies on awarding organisations to verify an individual has achieved required qualifications before CSCS issues the appropriate card.
"CSCS is confident the vast majority of cards issued are a result of a legitimate qualification."
Watch an extended report on Newsnight at 22:30 BST on BBC2