Two Yorkshire Labour leaders have teamed up with a Conservative from the Midlands to call on Ministers to "wholeheartedly reject" a report by the National Infrastructure Commission (NIC) on how major rail schemes could fit together.
And Transport for the North, the body set up to transform northern transport links, say the options in the report "represent a levelling down of ambition and investment, compared to the commitments made by government to level up".
Judith Blake, Dan Jarvis and Kay Cutts, who lead the Connecting Britain campaign lobbying for the Eastern leg of HS2, say the scenarios laid out in the NIC's Rail Needs Assessment from last month "would seriously inhibit economic growth and rebalancing".
Ahead of the Government's Integrated Rail Plan being published this year, the commission recommended that investment should be focused on boosting mainline services across the North and the Midlands.
Its assessment stated it would be “potentially cheaper and faster” to deliver improvements to regional journeys through a combination of new lines and upgrades, rather than Phase 2b of HS2.
This could mean the controversial high speed rail scheme would stop at East Midlands Parkway on the edge of Nottingham rather than carrying on to Sheffield and Leeds.
Coun Blake, the leader of Leeds city council, Mr Jarvis, the mayor of the Sheffield City Region and Coun Cutts, the Conservative leader of Nottinghamshire County Council, say the NIC report is based on "flawed assumptions".
They write: "We are calling on the government in the forthcoming Integrated Rail Plan (IRP) to confirm a clear timescale that honours your commitment and sets out when the Eastern leg will be delivered in full and at pace, to allow for our cities and towns to have certainty around their growth strategies and long-term economic planning.
"Each year of delay costs the North and Midlands £4.9bn. We need to see the hybrid bills for HS2 East in Parliament by 2023, and work can also begin on sections that do not require legislation.
"The North and Midlands stand ready to work with government to review the current delivery arrangements for HS2 to ensure the most effective integration between the Eastern leg in full with NPR, MER and related upgrades.
"We are calling for the establishment of an arms-length delivery body for High Speed North and Midlands, to ensure the best engineering solutions for those areas, full value for money and the greatest potential for the economic rebalancing this country so badly needs."
The future of HS2 has been the subject of fierce debate due to its rising costs and environmental impact, though in February Boris Johnson committed to building the route in full following the findings of the Government's Oakervee review.
In a separate letter setting out its statutory advice to Transport Secretary Grant Shapps, Transport for the North chief executive Barry White calls for a strong government commitment to HS2 as well as Northern Powerhouse Rail, the proposed high speed rail scheme connecting the big cities of the North.
He wrote: "After 10 years we are still debating what is required for HS2 whilst 7,500 miles of High Speed railway is being built around the world.
"The Government’s commitment will give Northern leaders and businesses confidence they can plan for future investments, as well as bolster our supply chains; SMEs; and freight and port operators, boosting inward investment, productivity and international competitiveness for the UK."
A Department for Transport spokesman said: “We’re committed to bringing the benefits of high speed rail to the East Midlands, Yorkshire and beyond.
"Our Integrated Rail Plan will outline how projects, including HS2 and Northern Powerhouse Rail, can work together to deliver the reliable train services that passengers need and deserve, as quickly as possible.”