An agency raid yesterday found 28 Indian men with no legal right to work in the UK.

Sisk has been served with a civil penalty notice for employing illegal workers. If they are unable to prove to the agency that right-to-work checks were carried out before giving the men jobs, a fine of up to £10,000 will be imposed for each worker.

But the accusation has stunned Sisk bosses who deny the charges completely.

A company statement said: “John Sisk & Son Ltd completely refutes the UK Border Agency’s assertion that it is employing illegal labour on the Pembroke site.  We are certain that none of those arrested works for Sisk, but rather for subcontractors working on the site.

“John Sisk & Son Ltd has robust policies to ensure that employees working for it and for subcontractors comply with all relevant UK labour laws, including the Pink Book, and that everyone carries a valid CSCS card.

“As a respected contractor in the UK, John Sisk & Son Ltd condemns the actions of those responsible for breaking the law and the terms of their contract in bringing illegal labour onto the site.  Sisk will support the UK Border Agency in robust action taken against them.”

Following the raid, eleven remain in immigration detention pending deportation, one man has been arrested by Dyfed-Powys Police on suspicion of using a false identity document and 16 have been placed on immigration bail.

The men must report weekly to a police station while the UK Border Agency secured the emergency travel documents necessary to deport them.

Earlier this year there were large demonstrations at the entrance to the site by people demanding construction jobs should be given to British workers.

Work on the £900m power station, which will be run by RWE npower, started in May 2009 and is due to last three-and-a-half years.

Alstom is the main contractor and Sisk the main civils contractor.