https://www.bbc.co.uk/news/business-48284105HS2 will not offer value for money and risks "short changing" the North of England, a group of peers has warned.
A report from the House of Lords Economic Affairs Committee also said it was "far from convinced" the new high-speed railway will be built within the £55.7bn budget.
It said the project should not go ahead without a new assessment of its costs and benefits.
The government said it "fundamentally disagreed" with the report.
The committee, which includes former chancellors Lord Darling and Lord Lamont, said that more than £4bn had already been spent on the first phase of HS2, which will run between London and Birmingham.
But it said the scheme had put too much emphasis on cutting journey times and not enough on the economic impact on regions.