The government-owned firm behind HS2 is to release a series of documents it previously admitted could lead to ministers scrapping the planned rail line if they were made public, The Sunday Telegraph can disclose.

HS2 Ltd said it will hand over papers detailing discussions with Theresa May's Government over the scheme's planned completion dates, following a two-and-a-half year legal battle by a GP who fears a "cover-up" took place to mask delays.

The move came ahead of a tribunal hearing next month and hours after The Sunday Telegraph approached the firm with a series of questions about the papers after establishing that it had admitted, as part of the case, that the information could be used by critics to successfully lobby for the "cancellation" of the project.

Dame Cheryl, Gillan, the senior Conservative MP, said: "It seems questionable that they have only at the eleventh hour said they are going to release the information, particularly now after you have been asking questions. But the very fact they have said they haven't been releasing this partly because it may help critics secure a cancellation is very alarming."

Separately, this newspaper understands that the firm, which is sponsored by the Department for Transport (DfT), has targeted whistleblowers with letters threatening legal action, following leaks relating to the project's budget and planned finish date.