Union officials have accused Transport Secretary Chris Grayling of “running scared” after he pulled out of a conference on transport in the North.
Mr Grayling was due to speak at the summit in Manchester on Monday but will now take part in a Parliamentary debate about a third runway at Heathrow.
RMT general secretary Mick Cash said the Mr Grayling had “chickened out”.
The Department for Transport said another minister, Baroness Sugg, would attend in Mr Grayling’s absence.
Mr Grayling was due to join Northern leaders including Greater Manchester Mayor Andy Burnham and Steve Rotheram, mayor of the Liverpool City Region, at the Northern Transport Summit.
Wigan MP Lisa Nandy said Mr Grayling’s cancellation “says everything you need to know about the contempt with which this government holds the north of England”.
Ms Nandy, who said on Wednesday the government knew Northern rail passengers would face disruption two years ago, said: “It’s unbelievable that Chris Grayling has decided to prioritise dealing with issues in London around Heathrow rather than coming here to the North of England.”
The RMT said it would hold a demonstration at the summit over its ongoing dispute with Northern over driver-only operated trains.
Mr Cash said: “He’s scared of the passengers he’s hung out to dry and he’s scared of the staff whose safety-critical jobs his franchise shambles have left facing the axe.”
A Department for Transport spokesman said: “The Secretary of State is not attending the summit as he is in London for the National Policy Statement in Parliament, a hugely significant vote that will benefit the whole country, including the North West.”
Meanwhile, a group of Greater Manchester MPs have written to Mr Grayling to say they are “minded” to vote in favour of a third runaway at Heathrow, but they want “firm assurances” that the government will invest in transport in the North.
The cross-party group of 20 MPs said it was “vital” that any approval for Heathrow expansion “does not impinge on the pipeline of the Northern Powerhouse projects”.