The transport secretary says the government will not "betray" the North by abandoning HS2 beyond Birmingham.
Chris Grayling reiterated his backing for lines to Manchester and Leeds after the government's commitment to go past the Midlands was called into question.
Regional transport bodies feared parts of the government were developing cold feet over HS2 because of cost concerns.
They urged Mr Grayling to win over sceptics before the project returns to parliament next year.
Mr Grayling was speaking at an event arranged by business leaders and MPs representing the North and Midlands who had grown concerned at the growing opposition to the project among senior Conservative figures.
He told the meeting that HS2 north of Birmingham was "vital" and the government would not "abandon its ambition".
"HS2 is a project that will benefit the whole country and we are committed to a second stage between the West Midlands and the North," he said.
The Department for Transport described claims the Northern sections of HS2 might not happen as "completely inaccurate".
Only the first phase of the £56bn project between London and Birmingham has so far been approved and is due to open in 2026.
Henri Murison, director of the Northern Powerhouse Partnership, said it was crucial the northern phases of HS2 were completed alongside the proposed Northern Powerhouse Rail - an East-West rail link between northern cities - if the government was to unlock the full economic potential of the project and allow cities like Bradford to benefit.
"We want the government to start discussing not if but when the North will see the full economic benefits of rail investment," he said.
Phase 2a connecting Birmingham to Crewe is due to be given final parliamentary approval during 2019 and is scheduled to be complete by 2027.
Legislation surrounding the final phase including two branches linking Crewe to Manchester and Leeds - due to open in 2033 - faces opposition when it is debated in parliament next year.
Joe Rukin, campaign manager of pressure group Stop HS2, said the project should be scaled back because costs had begun to rise before significant progress has been made on the first section.
"Nobody knows how much this project is going to cost," he said.