The former HS1, Crossrail and Eurostar boss, Rob Holden, said he applied to work on the high-speed rail project but was rejected by a “senior official” in the Department for Transport.
Mr Holden said he was surprised by the decision as his work on HS1 saw it “come in one time and under budget”.
His comments come as Rishi Sunak prepares to scrap the northern leg of HS2 from Birmingham to Manchester, weeks after The Independent revealed secret plans to do so.
The former HS1 chief executive said HS2 was always “ill conceived” because it was designed to run at an “inappropriate speed” for the UK: originally 400km/h, since cut to 360km/h.
Mr Holden said most high-speed lines in Europe run at 300km/h, with the costs of going much faster becoming “exponentially higher”.
The costs of HS2 in particular have spiralled because of “a lack of control”. He said: “That arises because of the extra commitments which are made during the process, very often the costs rise because the design of a mega-project like HS2 is not as complete as it should be.
“The overall cost of the railway should never have been as much as it was, as I say, because it was designed to operate at a speed which is not appropriate for this country.”
Mr Holden also said HS1 was successful because the true budget was known to just “a handful of people”, while HS2 contractors inflated their prices once they saw the latter project’s true budget.
On Tuesday morning, a defiant Mr Sunak continued to dismiss questions about the future of HS2’s northern leg as “speculation”, despite reports he has already decided to scrap it.
The prime minister said he is “looking at the facts” and would take his time to “get the decision right”, citing his move to push back some of Britain’s net zero targets.
“That’s what I will do with this, as I do with everything else,” he said.
It comes weeks after The Independent’s bombshell revelation of secret talks – codenamed Project Redwood – about scrapping the rail line to save cash.
In a tetchy interview with BBC Breakfast, Mr Sunak repeatedly refused to confirm whether the link from Birmingham to Manchester had been shelved.
He said there are “spades in the ground” but added that he would “not be forced into a premature decision because it’s good for someone’s TV programme”.
Mr Sunak said: “What I want to do is make the right decision for the country.
“This is an enormous amount of people’s money, taxpayers money … billions and billions of pounds. We shouldn’t be rushed into things like that.”
The idea of axeing HS2’s northern route has been opposed by three former prime ministers – Boris Johnson, Theresa May and David Cameron – as well as former chancellors George Osborne and Philip Hammond. Mr Johnson said it would be “utter madness”.
And on Monday Andy Street, the Tory mayor of the West Midlands, pleaded with Mr Sunak not to ditch the northern leg of HS2. The influential figure did not rule out resigning over the issue and said it would mean “cancelling the future” and warned that Britain’s credibility in the eyes of global investors was “now at stake”.