CIMIC's construction subsidiary CPB Contractors has been told by Queensland regulators to prove it has enough cash to keep building projects like Brisbane's $5.4 billion Cross River Rail after concerns were raised over its financial viability.

The Queensland Building and Construction Commission (QBCC) is understood to have sent a letter to CPB Contractors on Wednesday asking why its license to operate shouldn't be suspended after the construction group refused to provide a Minimum Financial Requirement (MFR) report when its net tangible assets dropped below 20 per cent.