The HS2 high-speed rail project is “essential” for the UK’s future and the potential gains “significantly outweigh” any risks, MPs have said.
The Commons Transport Committee also said the estimated cost of up to £50bn had been exaggerated, leading opponents to think ministers were offering a “blank cheque”.
And extending the link to the north of England should be speeded up, it added.
But the Stop HS2 group called the report “a cheerleading whitewash”.
HS2 would cut journey times between London, the Midlands and the north of England.
The first phase, from London to Birmingham, is due for completion in 2026, with a second Y-shaped section from Birmingham to Manchester and Leeds due to be finished in 2032-33.
The High Speed Rail Bill, which, with its supporting material, runs to almost 50,000 words, is currently before Parliament.
The committee said serious thought should be given to building the two phases at the same time.
It said incoming HS2 Ltd chairman Sir David Higgins should report to ministers by the end of next year “on options for speeding up HS2 so that trains run north of Birmingham on high-speed routes well before 2032-33″.
The cost of the project in its entirety is estimated at £42.6bn, with £7.5bn needed for the high-speed trains. Of the £42.6bn, a total of £14.56bn is contingency.
The committee said: “The Department for Transport’s (DfT’s) communications about HS2 should emphasise that the estimated cost is £28bn, not £50bn, 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.”
It added: “The project is now commonly regarded as costing £50bn and rising. This has led to exaggerated references to HS2 requiring a ‘blank cheque’ from government.”
The MPs heard from bosses of consultants KPMG, who said in a report that by 2037 HS2 would boost the UK economy by £15bn a year.
Some economists who gave evidence cast doubt on this figure.
In its report, the committee said the DfT should “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″ and suggested analysis should be “further developed”.
Its support for HS2 “was not unqualified”, it added, saying it remained concerned about how Heathrow would be incorporated into phase one and what impact including a stop at the airport would have on the budget.
The report said the committee was “convinced that it is essential for the UK for HS2 to go ahead”.
But 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 £50bn, the committee want to pretend it is £28bn, even though they said it would be £34bn 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.
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.”