More than 40 Conservative MPs have written to the prime minister to urge him not to cancel HS2, exposing deep divisions within the party over the future of the new railway line from London to the north of England.

At least 30 MPs, including former international development secretary Andrew Mitchell, signed a joint letter on Tuesday night that calls on Boris Johnson to deliver the “long overdue” rail line in full, despite costs apparently spiralling to £106bn, arguing it will be a “key engine for growth that we must not waver from”.

“Abandoning the project would leave the north reliant on aged Victorian infrastructure,” the MPs warn in the letter, which has been seen by the Guardian. They want Johnson to commit not just to HS2 but also Northern Powerhouse Rail (NPR), a new £39bn east-west line linking Liverpool to Hull, which has been designed to connect with HS2 and would use 80km of HS2’s track.

A further dozen Conservative MPs have written their own letters to Johnson confirming their support, according to one signatory of the joint letter, which comes a month after 21 anti-HS2 Tory MPs wrote to Johnson to tell him they had formed an “HS2 Review Group” to oppose the project.

Some of Johnson’s senior advisers are known to be against HS2 and have allegedly been briefing strongly against the project. But one new Tory MP denied that two of the prime minister’s closest aides, Dominic Cummings and Andrew Gilligan, had been trying to turn the new Westminster cohort against HS2.

“There’s been no effort to talk me out of supporting HS2,” said one, who won his seat from Labour in December and signed the latest letter. “Andrew Gilligan rang me to ask me my top five transport priorities and asked me for my views on HS2 but he gave no view either way.”

Timeline HS2 - over-budget and behind schedule Show High Speed Two Ltd is set up by the Labour government to examine possibilities for increasing high-speed rail capacity in the UK. The project is split into two phases - London to Birmingham forms phase one, with phase two extending the route to Manchester and Leeds. The transport secretary, Conservative Justine Greening, announces the decision to build HS2. A judicial review is called into the HS2 decision. Lord Justice Ouseley upholds one of the 10 grounds for complaint about HS2 in the judicial review – the claim that the government had acted unfairly and unlawfully when consulting on compensation for homeowners affected by the route. The Conservative prime minister, David Cameron, describes the project as "absolutely vital" as MPs approve funding. The high-speed rail (London-West Midlands) bill is formally introduced in parliament. After freedom of information requests, a 2012 Department for Transport viability report into HS2 is released, revealing the department considered it unaffordable. Allan Cook replaces Sir Terry Morgan as chair of HS2, after the latter fails to deliver the opening of the Crossrail project in London on schedule. A report from the New Economics Foundation suggests HS2 will deliver the most benefit to London, and exacerbate regional inequality. A fresh government review begins into HS2 into whether the scheme should be approved, amended or scrapped entirely. The Conservative transport secretary, Grant Shapps, announces that full HS2 services between London and Birmingham will be delayed by up to five years to 2031, and that the final completion of the northern section of the high-speed rail network would likely be delayed by seven years until 2040. He also confirmed the budget had escalated from the official £56bn at 2015 prices to up to £88bn at today’s prices. After a period of review, prime minister Boris Johnson announces that HS2 will go ahead, alongside a package of measures aimed at improving bus and cycling links outside of London.

In their new joint letter, the MPs warn Johnson a “false choice” should not be created between HS2 or NPR. “Both projects are vital for the development of the north, which has for too long been the poor relation when it comes to infrastructure,” they argue. They say they accept some changes might need to be made but that both new lines were “vital for the economic future of the north and Midlands”.

They add: “Major infrastructure projects often face scepticism at the outset. We need to have a bold and decisive vision for our country and be ambitious about what we can achieve. Delivering HS2 will allow us to do just that.”

It is understood the new letter was masterminded by Kieran Mullan, the new MP for Crewe, which will have a HS2 stop. It was signed by other new MPs including Robert Largan, who won the Derbyshire seat of High Peak, on the Greater Manchester border; Jonathan Gullis (Stoke-on-Trent North); and Darren Henry (Broxtowe, Nottinghamshire). Other signatories included Mitchell, who represents Sutton Coldfield in the Midlands, and former northern powerhouse minister Andrew Percy (Brigg and Goole).

Q&A How much work has already been done on HS2 - and how much has it cost? Show As of March 2019, £7.4bn had been spent. Much of the work done so far has been on paper: detailed engineering designs of the length of the route, years of public consultation, and legislation. Officially launched by Labour in 2009, HS2 was reviewed and tweaked by the coalition government and green-lighted in 2012. So far money has been spent on detailed engineering designs of the length of the route, years of public consultation, and legislation. Buying land along the route, either for direct demolition or to relieve blighted homeowners, has accounted for a large total of the £7.4bn invested to date. In terms of physical activity, so-called preparatory works have started, although no track or tunnel has yet been built. At the southern end, HS2 has demolished housing estates, parks and office blocks around Euston, and started moving tens of thousands of skeletons out of the way. Train depots and industrial estates have been razed to build another HS2 station at Old Oak Common, while brownfield sites in Birmingham have been levelled for the Curzon Street station and approach. According to HS2, work has taken place at 250 sites, including archaeological digs and tree planting as well as construction. Gwyn Topham Transport correspondent

The HS2 Review Group, which opposes the project, is chaired by North Oxfordshire MP Victoria Prentice, whose constituency is bisected by HS2. Other members include James Grundy, who now represents the former Labour stronghold of Leigh in Greater Manchester; Alexander Stafford, the new MP for Rother Valley, in South Yorkshire; and Andy Carter, who won the Warrington South seat.

They argue the money should instead be spent on local transport improvements, with some arguing their constituents will experience all the disruption but none of the benefits of HS2. “To deliver for the vast majority of people in the Midlands and the north, we need to scrap that white elephant project and concentrate the resources on local connectivity, which will have an impact and benefit for the many and not just for the few,” said Andrew Bridgen, MP for North West Leicestershire, which will be crossed by 18.6km of HS2 track with no stops.

A leaked copy of a report into HS2, written by former HS2 chairman Doug Oakervee, suggested costs had risen from a maximum of £88bn four months ago to at least £106bn. The report, obtained by the Financial Times, apparently also recommended that the second leg of the line north of Birmingham be paused for six months to investigate its value for money.

Under current plans, HS2 is to be built in a ‘Y’ configuration. London will be on the bottom of the ‘Y’, Birmingham in the middle, Leeds at the top right and Manchester at the top left. Almost £9bn has already been spent on the project so far, with significant land purchases and clearances well underway in Birmingham and around Euston in London. Phase one, from London to Birmingham, is said to be “shovel ready”, with the first track set to be laid in spring if Johnson gives it the green light.