Network Rail has rubbished suggestions that the HS2 high-speed rail link could be replaced with upgrades to existing lines, warning that passengers would face “absurd” levels of disruption from such an approach.

The intervention by the rail infrastructure manager comes amid reports that Boris Johnson’s government is looking at ditching parts of the line and replacing it with upgrades – under pressure from anti-HS2 campaigners.

“It’s one of the things that opponents of HS2 are very good at understating,” chief executive Andrew Haines told the Construction News trade magazine.

“We did some work recently on just how much disruption you would need to do on the East Coast Main Line to do significant capacity improvements if you didn’t do HS2 phase two. I think [the timeframe for how long work would last was] something absurd.

Mr Haines added: “It’s very easy to talk about alternatives to HS2 but the disruption of doing that [would be great].”

A previous Network Rail study found that Britain would effectively be left without a functioning intercity rail service at weekends for at least 15 years if it tried to match HS2’s capacity boost by upgrading existing lines.

The 2013 study found that delivering the same capacity boost to the existing network using upgrades instead of building HS2 would mean more than 2,770 weekend closures over a 15-year period.

HS2 is a high-speed network with three lines radiating from Birmingham to London, Manchester, and Leeds. Trains will also be able to run for part of their journey on existing lines to reach other destinations such as Edinburgh, Newcastle, Stafford and Sheffield.

As well as cutting journey times between large cities the line will free up capacity on existing mainlines for more regional and local services, which currently compete for space with fast intercity trains.

Network Rail’s study found that trying to replicate HS2’s benefits to existing lines with upgrades would require three huge concurrent projects to rebuild the west coast mainline, midland mainline, and east coast mainline. The upgrades would only deal with capacity and not delivery any of the journey times benefits of HS2, or the capacity of passengers actually carried on HS2 itself.

Any delays would also mean closures would also drag on even longer: a previous upgrade to the West Coast Main Line at the beginning of the century was delayed by several years and ran several times over budget.

Network Rail confirmed to The Independent that it had drawn up a new “advisory” report for the government alluded to by the body’s chief executive in his latest comments.

A Network Rail spokesperson said: “Network Rail, as experts in capacity and planning, is helping government to understand the capacity challenges on Britain’s railways and the many options that exist and their potential costs, timescales and consequences.”

A decision on whether to go ahead with HS2 is expected in the next few weeks, as the project comes under more pressure aiming rising cost projections.

A leaked version of the Oakervee review commissioned by Boris Johnson into the line says costs for the whole scheme could swell to £106bn – though it recommends going ahead with at least the first phase because of the project’s benefits.

Under threat: the planned High Speed 2 network (HS2)

The review also reportedly says the second phase from the West Midlands to Manchester and Leeds should be paused for six months to examine whether the upgrade could be delivered by a mixture of upgrades and new line.

Grant Shapps, the transport secretary, told Sky News on Monday that the decision on the line was “massive” and that it “needs to be fact based”.

He said he had “a relatively neutral point of view, adding: “We’ll be making a final decision, along with the prime minister and the chancellor, on this very shortly.”

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Stop HS2 campaign manager Joe Rukin said: “It has been absolutely clear from the outset that HS2 is an unaffordable white elephant that has only got this far because it’s been lobbied for by those with a vested interest in building it, and those idiot politicians who have fallen for their spin and like petulant children keep demanding that it’s not ‘either or’, and that they want everything and the want it now.”

Businesses have however reacted with alarm to suggestions that the project should be scrapped or curtailed. Adam Marshall, director general at the British Chambers of Commerce, said: “Our business communities want to see HS2 delivered in full without further delay.