The project, which could cost as much as 34 billion, was "a massive, massive gamble", said Stop HS2 convener Joe Rukin.
The scheme, involving a London-Birmingham line and then links to Scotland, had not been properly assessed, Mr Rukin told the Commons transport committee. He said: "The more we find out (about) the project, the worse it seems to be."
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Hide AdMatthew Farrow, the CBI's transport head, said the body supported HS2 as there were concerns about overcrowding on the London to Scotland west coast main line.