The company building the HS2 rail line between London and Birmingham says it is investigating "allegations" about the practices of some "labour suppliers" to the project.
HS2 Ltd is looking into two firms supplying workers to build the West Midlands section of the high-speed line.
It is understood to relate to billing for the staff supplied.
The Department for Transport said it would ensure "any claims of wrongdoing in HS2's supply chain" were "thoroughly" dealt with.
The existence of HS2's investigation, which began earlier this year was first reported by the i newspaper on Monday.
According to the paper, the matter relates to misclassification of staff and claims that inflated rates were being charged for them.
It is understood the two firms being looked at were providing workers to Balfour Beatty Vinci, one of HS2's contractors.
A HS2 spokesperson said it treated "all whistleblower allegations seriously", adding that Balfour Beatty Vinci "had implemented additional monitoring and controls".
It is understood one of the labour suppliers remains suspended from new contracts while inquiries continue.
A Department for Transport spokesman said the government and HS2 "take all whistleblower allegations extremely seriously and have a zero-tolerance attitude towards fraud, bribery, and corruption".
"We will therefore ensure any claims of wrongdoing in HS2's supply chain are thoroughly investigated."
The police and HMRC are not believed to be involved.
