Officials have been warned there is no alternative to building a brand new rail for services between Birmingham, Manchester and London.
Network Rail, the public body in charge of the rail network's infrastructure, said the current West Coast Main Line could not provide the number of rail services needed for projected passengers journey even if it were upgraded.
It said it had been warning of this very point since 2009 when the then Labour government first unveiled its ideas for HS2.
The warning is contained in a letter from Network Rail's chief executive Andrew Haines to the Department for Transport in which he said there was no "viable alternative" to the HS2 high-speed line.
Phase one of HS2 will run between Birmingham and London and is set to commence services by 2031 while the second phase will connect Birmingham with the North West, East Midlands and Yorkshire and is due for completion by 2040.
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It has never been far from the headlines in recent months with campaigners and some MPs calling for it to be scrapped while supporters say it is vital to rebalancing the nation's economy away from London.
The line is intended to address capacity issues on the West Coast Main Line.
The results of a review by its former chairman Douglas Oakervee are due to be published soon although a recent leak suggested the report backed continuing with phase one but elements of phase two should be looked at again.
Mr Haines also warns that scrapping HS2 would mean upgrades were required on the East Coast Main Line to allow it to cope with passenger numbers, causing significant disruption that could last up to 29 years.
He said it was not possible to upgrade the West Coast Main Line to provide the capacity required.
"There are only ten per cent more seats today in the busiest hour of the day into London Euston than there were in 2008," he said.
"Crowding is a particular and growing problem for people commuting from places like Northampton and Milton Keynes.
"On the West Coast Main Line, we concluded in our 2009 new lines report that a new line in each direction was the only viable way of meeting forecast growth in passenger demand.
"We forecast (at that time) to be two per cent a year (30 per cent in the busiest hour 2007-2020).
"Since then, we have seen growth of double that - and up to 5.4 per cent a year (70 per cent in the busiest three hours of the day 2007-2017).
"Our new lines report concluded that the differential between available capacity and future demand on the line was so large that upgrades to existing infrastructure alone would not be sufficient.
"Only new running lines could meet long-term demand and that high-speed infrastructure would provide better value for money.
"The report concluded two new running lines were required on the West Coast Main Line from Manchester to London."
He said Network Rail and the Department for Transport had produced a study looking at alternatives to HS2 but had found there was no alternative to creating a new rail line.
