Passengers face up to three decades of travel chaos if the High Speed 2 (HS2) project is scrapped, officials warn.

Abandoning the £100billion proposal would force rail bosses to launch costly upgrades on other lines to tackle overcrowding, causing 'significant' disruption to weekend services across the country, according to Network Rail.

And it is estimated these schemes, which would create only two-thirds of HS2's extra seating at best, could take up to 29 years on the worst-affected route from London to Edinburgh.

Threat: Abandoning HS2 would force rail bosses to launch costly upgrades on other lines to tackle overcrowding, causing 'significant' disruption to weekend services across the country

HS2 is forecast to open to passengers in 16 to 20 years' time.

Network Rail has also told the Government that plans to upgrade the North's rail network, a key election pledge of Boris Johnson, would be £15billion more expensive without HS2.

The authority's chief, Andrew Haines, issued the warning to the Department for Transport (DfT) in a letter dated January 14 which has been seen by the Mail.

He added that junking HS2 would force a 'fundamental' rethink of plans for Britain's railways and that alternatives would almost certainly face similar delays and complaints.

Network Rail was asked by Transport Secretary Grant Shapps to set out the arguments for keeping the controversial project as Johnson weighs calls to scrap it.

With ministers divided over the issue, the Prime Minister was expected to meet Sajid Javid, the Chancellor, and Shapps to hammer out a resolution this week.

A draft review of HS2 that was leaked last year said the scheme, a decade behind schedule, should proceed despite costs spiralling to an expected £106billion.

Business leaders and councils in the Midlands and the North say it is vital to support economic growth.

Warning: Network Rail boss Andrew Haines

But critics point to HS2's environmental impact and rising costs, originally estimated at £33billion, and say adding capacity on existing lines or building new ones would be better value.

In the letter to the DfT, Network Rail said only HS2 could realistically deliver desperately-needed extra capacity.

It said the West Coast Main Line, the East Coast Main Line and the Midlands Main Line were effectively full and that upgrading the routes would not deliver the same extra seating or journey times as HS2.

In the scenario that delivers the most benefits, or 66 per cent of HS2's capacity, 51 improvement projects would need to be launched.

Haines said: 'On the East Coast Main Line where, assuming continuous weekends of closures to complete the works, the route would be closed at one location every week for between 26 and 29 years. If closure at two locations were assumed, this would take 13-14 years.'

Network Rail said the improvements could be shortened to at least ten years if it is allowed three sets of works on each line simultaneously.

But it said there was no way to upgrade without significant disruption to weekend travel over a lengthy period.

Similar work a decade ago on the West Coast Main Line, which has not addressed capacity issues, caused long-running disruption, with businesses complaining that it cost them tens of millions of pounds.

This month, Birmingham mayor Andy Street and Manchester mayor Andy Burnham said: 'Modern railways such as HS2 and Northern Powerhouse Rail are the single biggest means to transform jobs and opportunities for people in the Midlands and the North.'

Joe Rukin, of campaign group Stop HS2, said: 'HS2 delivers capacity where it is needed the least, decades in the future at maximum cost. The real need for capacity is in short-distance journeys in and around left-behind towns and cities.'

Lord Berkeley, deputy chairman of a government-commissioned review into HS2, said improvements to existing mainline services would be a better use of public money than HS2.

'Quick wins on services in the North and Midlands are essential, as part of a 20-year programme of improvements for commuters,' he said.