The move follows a probe by BBC's Newsnight which revealed fraudulent activity taking place at five test centres where candidates were taking tests accredited by both CITB and the British Safety Council.
Overall, 4,615 cards have been invalidated.
It is understood 6,000 candidates were asked to retake the Health, Safety and Environment (HS&E) test while 2,000 applicants were requested to resit the Site Safety Plus certificate.
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While vouchers have been provided for those taking a retest, 5,480 applicants did not register for the resit and a further 533 have until 14 February to do so.
UCATT Acting General Secretary, Brian Rye, said the move was "good, decisive action".
"The CSCS card represents an industry standard and consequently must be protected," he said.
"UCATT supports the action of the CSCS and the CITB in acting quickly to restore the image of the CSCS card."
Graham Wren, chief executive of CSCS, added: "While the number of people suspected of fraudulent activity is relatively small, compared to the 400,000 cards issued each year, this type of behaviour has the potential to undermine legitimate employees and harm the reputation of the industry
"We are urging employers to check the validity of all cards by either electronically reading the CSCS SmartCard or by using "Card Checker" via the CITB website. It is important employers are able to trust the training and qualification providers."
(LM/MH)
