Prime Minister Rishi Sunak used his speech at the Conservative Party conference on Wednesday to confirm he has axed Phases 2a and 2b of the high-speed railway.
Mr Cameron said the decision would fuel the view that Britain is “heading in the wrong direction”, while Mr Osborne stated he agreed with that opinion.
Mr Harper told Sky News: “They’re absolutely entitled to their opinion. I worked closely with both of them in government and was very proud to serve with them. But that was a number of years ago.
This Government’s taken a different decision
“The facts have changed, the costs of the project have escalated, the patterns of travel have changed post-pandemic.
“So this Government’s taken a different decision. The Prime Minister’s taken a different decision that he thinks, and I think, is in the interest of the country.”
The Cabinet minister said “saving the £36 billion we were due to spend” on HS2 and “investing every penny” in alternative transport projects was “a decision I’m proud of”.
He told BBC Breakfast the decision was partly about the cost of HS2 “but also the change in behaviour post-pandemic”.
He said: “We’ve seen a big drop in the number of people using the railways, about 15-20% of passengers haven’t returned … and there’s been a 25-30% revenue decline.
“And the business case for HS2 was heavily weighted towards business travel. Business and commuter travel is 50% down where it was pre-pandemic.”
Mr Harper could not say “in detail” how much would be spent on paying off contracts that have been cancelled by cutting HS2’s northern leg.
Pressed repeatedly, he said: “We’ve made some assumptions. But we’ve also made some assumptions about the money we’ll recover from land sales, we think they will broadly balance out.”