The probe followed claims by a whistleblower that self-employed workers were being charged out at PAYE rates.

The allegations were made earlier this year and involved labour supplied to Balfour Beatty Vinci (BBV).

HS2 said an investigation is underway and BBV has “implemented additional monitoring and controls.”

Labour supply specialist Danny Sullivan Group is co-operating with the investigation.

The firm supplies several hundred workers across different HS2 sites which are continuing to operate as normal while the probe is underway.

A Danny Sullivan Group statement said: “We are aware of the allegations that have been raised in relation to HS2. As a business with a long and reputable history we take these allegations extremely seriously and are co-operating fully with the ongoing investigation.

“We have acted swiftly to respond to the information, commissioning a detailed and thorough internal review, which remains ongoing.”

A second company was originally suspended and investigated in relation to the allegations.

The suspension was lifted by BBV following a period of remediation and the company remains under enhanced monitoring.