Former Tory prime ministers have warned Rishi Sunak about "delivering a mutilated HS2" after his government refused to deny it was considering axing part of the project.
Boris Johnson said suggestions the Birmingham to Manchester route could be cut over cost concerns were "desperate" and "Treasury-driven nonsense".
He went on to urge the prime minister to deliver on the 2019 levelling-up pledge the Tories were elected on.
The paper quoted an ally who said it was "unusual" for the former prime minister, who resigned after the Brexit referendum result in 2016, to intervene in politics, but felt HS2 was "different".
"He thinks it's not only important in its own right - it's central to levelling up - but also that it's a totemic Conservative pledge," the anonymous ally said.
Lobbying by the former premiers comes as the government's infrastructure tsar warned cancelling the Manchester leg would be a "tragedy" and send an international message the UK was no longer a place to invest in major projects.
National Infrastructure Commission chairman Sir John Armitt acknowledged the need to control costs but told BBC Radio 4's Today programme: "If we don't continue, what are we saying to the rest of the world?
"What are we saying to all those investors that we want to bring into the UK? Here's a country which sets itself ambitions and then runs away when it starts to see some challenges. We have to meet the challenges."
Reports suggest bill has surpassed £100bn
The planned railway - announced by the last Labour government but backed by successive Tory administrations - is intended to link London, the Midlands and the north of England but has been plagued by delays and rising costs.
A budget of £55.7bn for the whole of HS2 was set out in 2015 but some reports suggest the bill has surpassed £100bn after having been driven up by recent inflation.
Ministers have already moved to pause parts of the project and even axed sections in the North - with the eastern leg between Birmingham and Leeds reduced to a spur line which is due to end in the East Midlands.
It was confirmed in March that construction between Birmingham and Crewe would be delayed by two years and services may not enter central London until the 2040s.
A government spokesperson said: "The HS2 project is already well under way with spades in the ground, and our focus remains on delivering it."