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The Government's proposed HS2 high-speed rail line relies on "laughable passenger forecasts" and is a "nonsense scheme driven by greed", the leader of a group opposing the project said.
HS2 would only benefit London and there was no economic or environmental case for it, added Lizzy Williams, chairman of the Stop HS2 organisation.
She was speaking as Stop HS2 held a convention at Stoneleigh Park in Warwickshire where speakers gave their analysis on the potential impact of the scheme.
Transport Secretary Philip Hammond's welcomed a report that spoke of the wide benefits of HS2 which will, initially, run from London to Birmingham.
But Ms Williams said: "The business case (for HS2) does not promise economic growth that will benefit the country. It is London who will be the winner overall.
"What about Wales, what about the South West, what about the rural economies? It relies on laughable passenger forecasts and takes no account of the changing world we live in."
She went on: "It is one rail solution to a rail problem and totally ignores the technological revolution that will enable more and more people to work without having to travel as much as they may do now.
"It also contradicts Treasury guidelines that investing in the existing networks should be the priority."
Residents' groups and some local councils are against HS2 which will pass through areas of natural beauty.
Ms Williams said: "Why are we considering destroying thousands of acres of productive farmland, meaning we will have to import more? Why are we planning to run this thing through major flood plains along the line?
"The entire scheme is nonsense, driven by greed and the lack of a robust plan for sustainable growth."
Every MP was invited to the convention.
Some Conservative MPs are opposed to HS2, but earlier in February Tory MP Graham Evans angered Stop HS2 by accusing campaigners of being "a ragtag alliance of luddites and nimbys".