CJ understands that the industry is awash with workers carrying out skilled trades when they only hold green labourers' cards.
One union source estimated there are currently four times as many green cards in circulation as there are labourers' jobs. He added that half of all Eastern European carpenters working on UK sites only have green cards.
Backers of the CSCS scheme have been concentrating on establishing a fully-carded workforce. But many now believe the emphasis should change to ensuring workers have the right cards.
Workers only have to pass a basic safety test to get a green card. But other cards for skilled trades involve sterner checks for the proper qualifications.
ADVERTISEMENTOne industry source said: "The card is really just acting as a passport to site at the moment. Foreign workers in particular just get a green card then carry out skilled work they used to do back in their own countries.
"Contractors turn a blind eye because they can get skilled workers to start straight away just by issuing them a labourers' card."
There are currently more than 1.3m CSCS cards in circulation. Chief executive Brian Adams said: "There are more green cards in circulation than we would expect.
"The next step is to ensure that everyone has the appropriate card. It is not something that we can police. It is up to management at site level to check that people have the appropriate cards."
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