But Mr Cameron told Sky News: "You have to plan for these things. High Speed 1, linking us to the Channel Tunnel was a long time in planning but it is good to get on with this now.
"And, yes, these are difficult economic times, but I think that is precisely the time you should be planning for the future, working out how we link up the cities of our country, how we reduce journey times, how we spread wealth and prosperity around the country.
"We do need to rebalance the economy, it has been too dominated by the south and by certain industries and high-speed rail will really help to create a better balanced economy."
Pressed on whether Tory opponents could force him into a U-turn, Mr Cameron insisted the scheme would go ahead.
"This is going to happen. I have been a strong supporter right from the start, in Opposition."
https://www.gov.uk/government/organisations/department-for-transport(Department for Transport)