Andy Burnham has said he could take legal action over HS2, warning: "We are fighting back".
He said the railway line will have "massive implications" for the north, promising that Labour will build HS2 if it wins the election.
In an interview with GB News, the Mayor of Greater Manchester said the north is being "completely ignored" by the Government.
He asked: "Why does the money always run out in the north? Why are people here always treated as second-class citizens when it comes to transport by people in Whitehall?
"If HS2 had been built first in Manchester and was going down the country, do we think the London leg would be scrapped? Of course it wouldn't."
Burnham said Shadow Transport Secretary Louise Haigh confirmed to him that Labour will build HS2 if elected next year.
He added that the Government had to fund both Northern Powerhouse Rail across the north of England and HS2 to link with the south of England.
The Mayor of Greater Manchester said: "We have a commitment around Northern Powerhouse Rail which is the critical thing as far as I am concerned. We need both.
"Why should we be forced to choose between having an east-west line and a north-south line.
"London has never been forced to choose - it gets the Elizabeth line, it gets HS2. But it is always here where people are forced to choose.
"We think we need both if we are to set up the northwest for the rest of this century."
His comments came after the Government refused to rule out scrapping the northern leg of the line last week, which runs from Birmingham to Manchester.
When asked whether he would take legal action over the issue, Burnham said: "All options would definitely be on the table.
"We're we're getting stronger in terms of the voice of the northwest.
"We don't seek fights with Whitehall, but we know how to answer them back now and we're not just gonna just gonna lie down and accept the way Whitehall has always treated the north of England."
He added: "We are fighting back. We're getting organized. we're not going to take things lying down so they will be hearing from us."
Departments have an obligation to consult any parties impacted by any major changes, including the scrapping of HS2.
This means northern authorities could bring a judicial review case to the High Court, in order to force the Government to reconsider their decision.
Defence Secretary Grant Shapps said that ministers have to "respond to the circumstances", when asked whether the Government will scrap HS2.
This comes just one week before the Conservative Party heads to Manchester for the annual party conference.
Speaking to Sky, Shapps added: "We did not know there would be [a] coronavirus pandemic. "You must look at the sequencing of the big infrastructure cash that you spend. Any government that does not do that, any opposition who claims you do not need to, is not fit to govern this country."
But Housing Minister Rachel Maclean told GB News that "no decisions" have been made on HS2 yet.
He said the decision, which is currently under consideration, is "total Treasury-driven nonsense", adding that it makes "no sense at all to deliver a mutilated HS2".
Conservative Mayor of the West Midlands, Andy Street, said a decision to scrap the northern section of the project would represent the "most expensive white elephant in UK history", warning that it would be "a disaster for the country".
Speaking to GB News earlier this week, Housing Minister Rachel Maclean said: "I think more broadly, you can see this Government's commitment to connectivity and transport across the whole country.
WATCH NOW: Eamonn and Isabel discuss the cost of HS2
"We've put £33 billion into levelling up the north since this government has been in office.
"And we will continue to look at these projects that are going to boost that connectivity and economic growth all over the whole of the United Kingdom."
I've had reassurances from Shadow Transport Secretary Louise Haigh, who was in the region yesterday.
A source close to Haigh stressed that Mr Burnham was speaking in the context of the government keeping the commitment to HS2.
The source said: "Labour have said we want to see the government deliver HS2, but if the Conservatives blow a hole in its progress, no potential government could commit when we have no idea of the scale of the damage we will inherit from the Tories."