Unauthorised payments to HS2 staff worth £1.76 million were a "shocking waste of taxpayers' money", an influential Commons committee has said.
The Public Accounts Committee (PAC) expressed concern that the former boss of HS2 Ltd, Simon Kirby, has "not been held to account for his actions".
It urged the Department for Transport (DfT) and the company to address the issues which led to the enhanced redundancy payouts "as a matter of urgency".
The PAC heard evidence Mr Kirby failed to act on DfT orders that the redundancy scheme should be restricted to statutory levels.
In a hard-hitting report it stated that the payments proceeded because of "weak internal processes at HS2 Ltd" which stopped decision-making and scrutiny bodies from receiving accurate information.
The relationship between the DfT and the company "was not robust enough to prevent this", the committee added.
It called on both organisations to "carefully consider whether any further action can now be taken" against Mr Kirby, who left his role as chief executive in December 2016 and is now chief operating officer at aerospace giant Rolls-Royce.
Mr Kirby said: "I did not approve the payments at issue and deny any allegation of wrongdoing.
"I have had no contact from either the NAO (National Audit Office) or DfT on the audit of HS2's accounts and redundancy payments.
"I left HS2 in December last year and the decision to make senior managers redundant, and under what terms, was not made until after I left."
DfT permanent secretary Bernadette Kelly wrote to the PAC committee on November 16 to say that legal advice had suggested no further action could be taken against Mr Kirby.
Labour's shadow transport secretary Andy McDonald accused Transport Secretary Chris Grayling of having a "cavalier attitude towards the nation's finances" and called on him to "take responsibility and spell out what he will do to prevent any more public money being squandered".
Transport and Salaried Staffs Association leader Manuel Cortes described the PAC report as "damning beyond belief" and claimed "heads really must roll at the Department for Transport".
He said: "Simon Kirby did the honourable thing and jumped to avoid being pushed.
"But it is Transport Secretary Chris Grayling who must shoulder the blame for the mismanagement of a vital public infrastructure project by his department that has led to these millions being siphoned out of the public purse."
A DfT spokeswoman said: "We have made clear to HS2 Ltd in the strongest terms that we expect them to always meet their obligations and responsibilities to the taxpayer. The progress of HS2 should not come at the expense of proper control over public money.
"HS2 Ltd's chairman Sir David Higgins is confident that the company, under new chief executive Mark Thurston, will not let this happen again.
"We have accepted the recommendations of the GIAA (Government Internal Audit Agency) report in full. HS2 Ltd and DfT are taking serious and proportionate action to implement all of the report's recommendations by early 2018."