It is "essential for the UK" for the HS2 high-speed rail project to go ahead, a report by MPs has said.
The risks of not going ahead with the project "significantly outweigh the risks of doing so," added the report from the House of Commons Transport Committee.
The committee, chaired by Louise Ellman, Labour MP for Liverpool Riverside, also said serious thought should be given to building the second-phase northern section of the line at the same time as the first phase London-to-Birmingham stretch.
The committee said it remained "convinced that the project is justified" although the MPs added they would not accept a situation in which other vital transport projects were delayed due to HS2 funding requirements.
The cost of the project in its entirety is estimated at £42.6 billion with £7.5 billion needed for the high-speed trains. Of this £42.6 billion, a total of £14.56 billion is contingency.
In its report, the committee said: "The Department for Transport's (DfT's) communications about HS2 should emphasise that the estimated cost is £28 billion, not £50 billion, and that cost increases to date have largely been due to the decision to undertake more tunnelling and other work to mitigate the impact of the project on people living near the route."
The report added: "The project is now commonly regarded as costing £50 billion and rising. This has led to exaggerated references to HS2 requiring a 'blank cheque' from Government."
The first phase of the scheme, from London through Tory heartlands in the Chilterns to Birmingham, is due for completion in 2026, with a second Y-shaped section from Birmingham to north-west and north-east England due to be finished in 2032/33.
In their report, the MPs said the incoming HS2 Ltd chairman Sir David Higgins should report to ministers by the end of 2014 "on options for speeding up HS2 so that trains run north of Birmingham on high-speed routes well before 2032/33".
The MPs took evidence from bosses of consultants KPMG who said in a report that by 2037 HS2 would boost the UK economy by £15 billion a year.
Some economists who gave evidence to the committee cast doubt on this figure. In its report, the committee said: "The DfT's written evidence clearly identifies the limitations with KPMG's research, referring to 'challenging' forecasts and 'uncertain' results.
"However, in other contexts the department has referred to HS2 generating '£15 billion of economic benefits annually' without significant qualification.
"We recommend that the Government recognise the current limitations of the work undertaken by KPMG in making public use of the KPMG estimate of wider economic benefits arising from HS2.
"We look forward to this analysis being further developed, including by recognising the impact of other continuing and proposed rail investments."
Saying its support for HS2 "was not unqualified", the committee said it remained concerned about how Heathrow will be incorporated into the plans for phase one and what impact including a Heathrow stop would have on the budget.
The report concluded: "Many important local issues will be debated and resolved by the (HS2) hybrid bill committee and there will be detailed debates about many aspects of the project.
"However, we are convinced that it is essential for the UK for HS2 to go ahead, and to do so as a project which has the backing of all three major political parties."
Launching the report today, Mrs Ellman said: "We remain confident that construction of a new high speed line is the only way to deliver the step change in capacity on the West Coast Main Line needed to accommodate long-term demand for both passengers and rail freight.
"If we are to spread the benefits from HS2 as widely as possible, it is vital we improve links between the conventional and high-speed networks and bring forward projects to speed up journey times on the conventional network."
Joe Rukin, campaign manager for the Stop HS2 group, said: "Unlike the Public Accounts Committee and Treasury Committee hearings on HS2, it was clear that this inquiry was going to be a cheerleading whitewash when the Transport Committee only called people who support HS2 to give evidence.
"Despite the official cost of HS2 standing at £50 billion, the committee want to pretend it is £28 billion, even though they said it would be £34 billion in 2011. In saying this and telling the DfT they should abandon their standard assessments to improve the case for HS2, they are effectively ordering the Government to 'spin harder' on HS2."
Transport Secretary Patrick McLoughlin said: "HS2 will be a heart bypass for the clogged arteries of our transport system. We therefore welcome the Transport Committee's conclusion that the new North-South railway is the best long-term solution to increasing capacity and that alternative proposals would simply not cope with the predicted increase in demand.
"The committee also concludes that HS2 will not come at the cost of other transport projects and that the benefits to northern cities could transform the nation's economic geography. This chimes closely with steps we have already taken."
Shadow transport secretary Mary Creagh said: "Labour supports HS2 because we must address the capacity problems that mean thousands of commuters face cramped, miserable journeys into Birmingham, Manchester, Leeds and London.
"However, three years of Government delays and mismanagement has caused costs to balloon. Incompetent ministers have only recently launched the consultation on phase 2 of the route, despite the fact that it was being worked on when Labour were in government."