High Speed 2 bosses face questions over the appointment of their chief executive just days before MPs sign off on the first stage of the project.
Phase one of HS2 is expected to be greenlit within days of parliament’s return from recess on 20 February.
The project recruited Mark Thurston to serve as its new chief executive last month, with Thurston joining from CH2M Hill - the engineering delivery partner for phase one of HS2.
And Thurston replaced another former CH2M boss, Roy Hill, interim chief executive from October to January, leading some to question a “revolving door” between the operations.
“It just doesn’t pass the smell test,” one Conservative MP told City A.M.
“There is potential for an unhealthy relationship between the directors in this company and public money.”
TaxPayers' Alliance chief executive John O'Connell added: “When HS2 bosses are left in charge of budgets that most government departments could only dream of, it is vital that correct procedures are in place that allow full transparency and accountability on spending and the awarding of contracts.
“Unfortunately the revolving door between HS2 Ltd and CH2M means that serious questions need to be asked as to how contracts can be awarded fairly when HS2 are appointing their contractors as a CEO."
HS2 maintains that it used headhunters to find the best candidate to replace Thurston, while Hill had no role in the appointment process.
In addition, it says that Hill was subject to substantial restrictions in his time at HS2, forced to excuse himself from meetings that discussed CH2M, and restricted from reading minutes afterwards.
HS2 chairman David Higgins said: “We recognise the need to avoid any conflict of interest so [Thurston] will cut all links with his previous employer.
“They will be treated in the same way as any other supplier - no more or less favourably than that. All those who bid for work with HS2 have the right to know that they will be treated on the same basis and, as we have shown to date, we are determined that our systems are constructed in such a way as to deliver that equal treatment.
"And the board of HS2 regularly reviews this process, aided by independent advisers, to ensure that is the case."