Deputy Prime Minister Nick Clegg insists it is right to find out whether a tunnel under the Peak District is possible - even if it is not then built straight away.
A feasibility study is being launched to look at the £6bn idea, part of work aimed at improving transport links between Sheffield and Manchester and uniting the north.
Motorists welcomed the general idea of making journeys between the two cities quicker and easier, saying it was many years overdue.
But some criticism questioned whether a tunnel, which would be the longest in Europe, would ever happen and if it was a pre-election gimmick.
Sheffield Hallam MP Mr Clegg, who announced the scheme as part of a package of improvements, told The Star: “I was very cautious not to overstate it, I don’t know whether this tunnel will be built but it is right to find out if it is feasible.
“If it was created it would be right up there with the major Alps tunnels.
“I can’t say now whether it will ever be built as we haven’t completed stage one, of finding out whether it is feasible.
“If it is then maybe in the years to come, just think about the environmental benefits to the Peak District National Park, that is already very considerable as well as improving links between Sheffield and Manchester.”
Separate work to take place on the A57 and A628, part of the package announced earlier this week, is to include a bypass where the two roads meet at Mottram near Manchester.