Two companies have been hit with fines totalling up to £400,000 for hiring illegal workers after a dawn raid on a former paper mill.
Twelve men and a woman were arrested and will be removed from the UK following the 5am operation targeting the site.
They were all working as subcontracted labourers and steel-fixers, according to the Home Office.
The two firms are among the first businesses to be hit with recently increased penalties for employing illegal workers
They are Northern Irish concrete supplier FP McCann which was fined up to £225,000 for five contractors found to be working illegally.
Stockport-based Adana Construction Ltd will have to pay up to £180,000 for employing another four immigration offenders.
The raid on the Shotton Mill site in Deeside, north Wales, took place on Tuesday March 19.
Immigration Enforcement officers swooped on the former paper mill, now being turned into a containerboard factory, following allegations from members of the public.
Seven contractors, from India and Albania, were later taken into custody, while the others were bailed and are required to report to immigration officials.
Officers entered with permission from the site managers, who have since launched an internal investigation.
HM Inspector Ryan Moore, of the Home Office Immigration Enforcement section, said: “Illegal working causes untold harm to communities, puts vulnerable people at risk, defrauds the public purse and undercuts honest employers and jobseekers.
“Our teams will do everything in our power to clamp down on this damaging practice and hit those who cheat our laws in the pocket. This operation was a huge success and I thank our officers who executed it expertly.”
The raid came after the Home Office tripled fines for companies employing illegal workers.
From February, the civil penalty rose to £45,000 per worker for a first breach, up from £15,000, and to £60,000 for repeat breaches, up from £20,000.
Both companies were issued civil penalty referral notices pending a review of evidence from the site visit and company records.
