Workers found in possession of fake cards during a routine security inspection of the site.

The Metropolitan Police's fraud squad has launched a fresh investigation into the CSCS after workers on Arsenal's Emirates stadium were found in possession of fake cards.

It is understood that the police were called by CSCS management after the cards were uncovered during an inspection on the site of the flagship project, which is being built by Sir Robert McAlpine.

The discovery follows a police investigation in December, when six men were arrested in Luton after a raid uncovered almost 100 fake CSCS cards.

It also follows an investigation in February into an illegal immigrant working as a security manager on the stadium, who was caught selling fake Nigerian and South African passports.

It is understood that the cards were identified because they did not bear the CSCS security hologram. Details of the number of cards found, and whether they were found on workers employed directly by Sir Robert McAlpine or by one of its subcontractors, were unavailable as Building went to press.

Bill Jenkins, CSCS' secretary, said: "There were some fraudulent cards found. We have handed the matter to the police fraud squad."

A spokesperson for CITB-ConstructionSkills, the card scheme operator, said it could not comment on the specific findings on the Arsenal project as a criminal investigation was under way.

He said: "As the service provider of CSCS, we continually strive to prevent fraud by building on systems we've already introduced."

The discovery comes as the number of CSCS forgeries is rising. In December last year, a police raid on an address in Luton uncovered 91 fake skills cards, including 30 CSCS cards, and other forged identities. The raid followed intelligence from CITB-ConstructionSkills and involved liaison between police, the Department for Work and Pensions and the Health and Safety Executive.

CSCS Ltd, the owner of CSCS, and the CITB have been working on measures to improve the security of the card in the wake of concerns over identity fraud.

Measures under consideration include smart cards which could include microchips, biometrics, and on-site swipe systems.

Arsenal and Sir Robert McAlpine were unavailable for comment.