A government-commissioned report says the alternative to a new HS2 high speed rail link would see 14 years of weekend route closures and longer journeys.
The report, by Network Rail and Atkins, says upgrading existing rail lines would severely affect the East Coast, Midland and West Coast mainlines.
It says that would double travel times between Leeds and London.
The study will inform the government's updated business case for HS2, which will be presented on Tuesday.
It will be the fifth official presentation given by the government on HS2.
HS2 is designed to shorten journey times between London and the Midlands and the north of England, but has attracted stiff opposition from some quarters because of doubts that the cost - last estimated at £42.6bn - will bring sufficient benefit.
Those living along its route also fear the disruption it will bring to their areas.
A previous report in September, by the consultants KPMG, said the project would be markedly beneficial to the UK economy, and boost overall growth by 0.8%.
However, that report failed to convince critics and KPMG itself included a caveat which said it had made the "implicit assumption" that transport connectivity was the only supply-side constraint to business location, and that it had ignored other possible constraints to growth, like labour or land shortages.
Some critics of HS2 see a plan to improve the existing lines as a better use of public money. But the report says that, in total, this would require 2,770 weekend closures involving 144,000 hours of work.
The BBC's transport correspondent, Richard Westcott, says he has been told it would cost £20bn to upgrade the UK's three existing north-south train lines.
Modelling a typical weekend, the report argues that the journey time from London to Leeds could be increased by two hours and 10 minutes to more than four and a half hours while the work is going on.
A journey between Huntingdon and Peterborough would be doubled to an hour.
Atkins has also concluded that residential and commercial demolitions would be required.
Hilary Wharf, director of the campaign group HS2 Action Alliance, said the report was seriously flawed and that upgrading the existing lines would bring bigger benefits to more passengers.
"This government-funded report is a complete fabrication. The main alternative to HS2 involves longer trains and reduced first-class capacity to provide more standard class seats," she said.
"No work is required at Euston to deliver the necessary capacity increase. Work is only required at three locations on the WCML [West Coast Main Line], and this is comparable to the work being carried out on the route at present."
A government source said: "We need to do something because our railways are nearly full, but the alternative to HS2 is a patch and mend job that would cause 14 years of gridlock, hellish journeys and rail replacement buses.
"The three main routes to the north would be crippled and the economy would be damaged."
And the British Chambers of Commerce said changing tack now would be bad for businesses: "Politicians flirting with a U-turn on HS2 are guilty of electoral short-termism and opportunism of the worst kind.
"Business communities in dozens of cities and towns, from many parts of the UK, remain strongly supportive of HS2."
On Sunday, the Treasury Minister, Danny Alexander, told the BBC's Andrew Marr programme that he was "very confident" the HS2 high speed rail project would be delivered within its £42.6bn budget.