The new chief executive of HS2 has refused to apologise for the rail project spending £100 million on a "bat shed" intended to protect the animals from passing trains.

Mark Wild told MPs on Thursday that the planned building in Buckinghamshire was a legal requirement.

The structure was criticised by Sir Jon Thompson, the outgoing chairman of HS2, who said there was "no evidence that high-speed trains interfere with bats".

Dame Bernadette Kelly, the Department for Transport's most senior civil servant, told the public accounts committee on Thursday that she and the Treasury had "challenged" the building of the bat shed.

Yet they found it was "the most efficient remedy" for protecting the creatures, which are legally protected in the UK regardless of their conservation status.

Asked at a hearing if he wanted to apologise for the structure, Mr Wild, who took over as chief executive earlier this month, said he could "understand why that would raise public concern".

But he added: "The law says that we must mitigate damage, harm to protected species. I can't apologise for complying with the law. This structure is the most appropriate.

"It is an extraordinary amount of money but it is in the context of a scheme that is costing tens of billions and it's built for 120 years."

The Prime Minister separately claimed on Thursday that HS2 "is a case study of the last government on how to mishandle a major project, over budget, over time, cut back and almost a textbook example of what the last government got wrong when it came to big infrastructure projects".

Sir Keir Starmer told reporters: "That doesn't mean we should abandon infrastructure projects but it does mean we've got to do them differently and we've got to do them better."

Mr Wild's refusal to apologise comes after Sir Jon, the HS2 chairman, quit this week after the Government said the rail project's latest cost estimate of £66 billion was not an "accurate or reliable" figure.

Dame Bernadette told MPs on Thursday: "We do not currently have an agreed cost estimate now for phase one... I say with great regret, sitting before the committee, that is the situation."

She added that a new cost estimate would likely not be forthcoming until late next year.

Sir Jon was the one who revealed the level of spending on the "bat shed", as he described it at a railway industry conference in November.

So far £32.8 billion has been spent on HS2 since it was approved in 2012, with the latest estimates coming after years of warnings that its spending was out of control.