Transport Secretary Chris Grayling has come under fresh fire after backing London's multibillion-pound Crossrail 2 scheme days after announcing that rail electrification plans for Wales and northern England are being cut.
Mr Grayling said on Monday that he would work with London Mayor Sadiq Khan to ensure Crossrail 2, the proposed north-south rail line running across London between Hertfordshire and Surrey, was "fair to the UK taxpayer".
But the statement prompted Greater Manchester Mayor Andy Burnham to say on Twitter: "On Friday, Tories say they can't afford rail schemes in the North. On Monday, they find billions more for London. Are these 2 things linked?"
Liverpool City Region Mayor Steve Rotheram also weighed in, saying that while he did not "begrudge" the investment in London and the South East, there needed to be balanced spending to "support growth in the North as well".
Mr Grayling came under fire last week for scrapping long-awaited rail electrification plans, with Labour claiming he was "taking people for a ride" and breaking promises on upgrading the routes.
The Transport Secretary said using new "bi-mode" trains which run on diesel and electricity meant the move to abandon commitments to electrify the main lines from Cardiff to Swansea and Kettering to Sheffield was about modifying plans "where it makes a difference".
On Monday it was announced that Mr Grayling and Mr Khan had agreed the need for new infrastructure in the capital at a meeting last week, as well as a funding package which "works for both London and the rest of the country".
Mr Grayling said he was a "supporter" of Crossrail 2, but cautioned that "we have to ensure that we get this right" given the price tag.
Mr Rotheram said the news came as "a bit of a surprise given it wasn't included in the Conservative Party manifesto".
He added: "Sadly the juxtaposition of last week's cancellation of northern rail electrification schemes and this week's green light for Crossrail, suggests it is just going to be business as usual from this Government when it comes to transport investment."
Mr Burnham said: "This statement from the Transport Secretary will cause widespread anger across the North of England. With every day that passes the promise of a Northern Powerhouse becomes ever more distant.
"People here have had to put up with sub-standard rail services for decades and will simply not accept that spending billions more on London is the country's highest priority for transport investment.
"It will not escape people's notice that this commitment to London today comes just days after the Transport Secretary cancelled electrification schemes all over the country. It raises the question of whether taxpayers everywhere else outside of London are paying to make Crossrail 2 viable.
"It is interesting that these statements come after Parliament has risen, which is denying any real scrutiny of these decisions but when they come to Manchester for their conference I can assure them that they will not be able to avoid these questions.
"The time has passed where we can take these decisions lying down. I will today be contacting fellow metro mayors and council leaders across the North of England to consider how we best challenge this Government's continued neglect of transport in the North together. "
The capital may have to fund half of the Crossrail 2 project during its construction, Mr Khan and Mr Grayling said.
London has already shown it could foot the bill for half of the scheme over its life, but the pair said they want to see if the city could do the same during the construction period.
Mr Khan said he was "pleased" to have "reached an agreement to take this vital project forward".
The scheme is estimated to cost around £30 billion at 2014 prices and construction could start in the early 2020s and the railway could be open by 2033.
The line would run as far north as Broxbourne in Hertfordshire and as far south as Epsom in Surrey, passing through central London via places such as Tottenham Court Road, Victoria, Chelsea and Clapham Junction.
Plans to electrify the Transpennine route have not been scrapped, according to the Department for Transport.
A spokesman said major upgrades to the Manchester - Leeds - York route were currently being "designed and developed, to enable us to deliver better journeys for passengers from 2022".
He added: "Passengers expect high quality rail services and we are committed to electrification where it delivers benefits, but will also take advantage of new technology to improve journeys."
However, it has not yet been decided whether the entire line will be electrified, and an investment decision will be made next year on the upgrade.
Analysis by an independent think tank found the North of England would have received £59 billion more over the last decade, had it received the same infrastructure spending per person as London.
Luke Raikes, senior research fellow at IPPR North, described the findings as a "national scandal", and said: "In most other advanced countries, decisions about transport investment are made locally or regionally, where people really in the know about local problems decide exactly what's needed.
"But in Britain, our Whitehall-knows-best attitude leads to the capital being the government's default option for more funding."
A Department for Transport spokeswoman said the Government was investing £1 billion to improve rail infrastructure across the North of England, as well as £800 million on new road schemes.
"In spending taxpayers' money, it's vital for the government to deliver value for the whole country.
"So while we have agreed to work further with Transport for London on Crossrail 2, we have also said that London needs to pay half of the upfront construction costs and we have not committed any public funding yet."
Shadow transport secretary Andy McDonald said "investment in the capital shouldn't come at the expense of the rest of the UK" and warned the Government risked "increasing regional inequality".
Mr McDonald said: "It is right that London should receive the transport infrastructure investment it needs, but by pulling the plug on electrification works across the country the Transport Secretary risks undermining public support for Crossrail 2.
"People elsewhere in the UK will be asking why billions of pounds more should be spent on London while promised works are cancelled in regions already suffering from under-investment.
"With more than half of the UK's total spending on transport networks invested in London, the Government risks further increasing regional inequality if this commitment to Crossrail 2 isn't accompanied by a commitment to a Crossrail for the North and the electrification of routes and improved services across Britain."