High Speed 2 will add £15bn a year to the UK economy, according to a new report commissioned by the Department for Transport, as the government tries to rebuild its crumbling economic case for the massive infrastructure project.
The report, carried out by accountancy giant KPMG, claims HS2 would bring significant benefit to regions outside of London, particularly in the midlands and north.
However, it also adds that these benefits depend "on the ability of businesses and people to respond to changes in connectivity" and the methodology "makes the implicit assumption that transport connectivity is the only supply-side constraint to business location".
It follows a stinging report by MPs which accused the DfT of making decisions on HS2 based on "fragile numbers, out-of-date data and assumptions which do not reflect real life".
HS2, which the government says will cost around £50bn (£78.8bn, €59.4bn) but other estimates put at as much as £80bn, will create a new high speed rail network connecting London with the north and midlands to slash journey times between major cities and boost business in the surrounding areas.
Campaigners against the project reacted angrily to the KPMG report.
"Everyone knows that these are bulls**t figures, and to be honest it is really sad that the government is still all aboard this express train to disaster," said Stop HS2 Campaign Manager Joe Rukin, adding "the more they flog this dead horse, the more stupid they look".
Richard Houghton, spokesman for HS2 Action Alliance, said the rail project is a "white elephant".
"Betting £50bn on a single train line being the solution to the economic woes of the country outside of London really is irresponsible - and doesn't cover up for the lack of a coherent economic strategy for the regions," he said.
"It's time to accept the facts: HS2 is an unnecessary waste of money."
Patrick McLoughlin, the government's transport secretary, defended HS2 in a speech presenting KPMG's report.
"The point about HS2 is that you won't have to travel on it to gain from the better transport system and economic growth it will support," he said.