HS2 needs a major rethink and should not go ahead without a new assessment of its business case, a group of peers has warned.
A report by the House of Lords Economic Affairs Committee warned it is "far from convinced" the new high-speed railway will be built within the £55.7 billion budget.
Today Northern Powerhouse chiefs said the project must go ahead in tandem with North Powerhouse Rail plans .
The committee, which includes former chancellors Lord Darling and Lord Lamont, stated the cost of the project could be cut by reducing the speed of the trains and moving the London terminus.
It claimed construction of HS2 should have started in the north rather than the south, adding the Northern Powerhouse Rail scheme connecting major towns and cities in the region should be completed alongside the high-speed railway.
A number of questions it raised in a 2015 report "have yet to be answered satisfactorily", the group warned.
Phase one of HS2 is due to open between London and Birmingham in December 2026, before the railway is extended to Crewe, Manchester and Leeds.
HS2 trains will also serve locations on the existing mainline network, such as Liverpool, Newcastle, Edinburgh and Glasgow.
Lord Forsyth, who chairs the House of Lords committee, said commuter services in the north of England are "badly overcrowded" and the region is being "short-changed" by existing plans.
The former Scottish Secretary added: "The costs of HS2 do not appear to be under control.
"It is surprising therefore that the Government has not carried out a proper assessment of proposals to reduce the cost of HS2 - such as lowering the speed of the railway or terminating in west London rather than Euston - which the committee recommended in 2015.
"A new appraisal of the project is required. If costs overrun on the first phase of the project, there could be insufficient funding for the rest of the new railway.
"The northern sections of High Speed 2 must not be sacrificed to make up for overspending on the railway's southern sections."
The report is the latest in a series of warnings about the railway project, with concern over its finances and environmental impact.
Henri Murison, Northern Powerhouse Partnership Director said: “Northern businesses and leading Civic Leaders and Mayors agree we need both HS2 and Northern Powerhouse Rail.”
“The North will then have a high speed network, with more seats across the Pennines and more frequency, benefiting those from Liverpool to Hull and up to Newcastle as well as enhanced connectivity through Leeds, Sheffield and Manchester to Birmingham and London.
"It is vital for the Northern Powerhouse, and cancelling either project would be a betrayal to the cause of rebalancing our country.”
Lancashire business leaders have also called for the planned next phase, HS3, to serve Lancashire and not stop on the outskirts of Wigan.
Alan Welsh, policy manager of the North and Western Lancashire Chamber of Commerce, said: "Where construction should have started on HS2 is hardly relevant.
"We are where we are and there is a danger that the North is going to lose out again if HS2 stops at Birmingham and plans for Northern Powerhouse Rail are watered down.
"It should not be a case of 'either one or the other', we need investment in both schemes and there is no guarantee that any money saved on HS2 will be used for transport elsewhere in the north. HS2 is not just about better connectivity, it's about increasing capacity for people and goods and helping to relieve congestion on roads as well.
"It seems strange that amid mounting criticism of HS2 nobody is questioning plans for Crossrail 2 in the South East - a scheme that will cost double that of the Elizabeth line (Crossrail 1) which is still yet to open.
"This could potentially be the biggest threat to HS2 ever getting past Birmingham and I would urge people in the North to continue making the strongest case for HS2 or risk losing billions of investment altogether."
A HS2 Ltd spokesman said the project will "generate around £92 billion in benefits to the UK economy".