Boris Johnson said he wants to reduce the bill for HS2 but that he would "hesitate before simply scrapping" the rail project.
The prime minister said the government would check the money invested in the multi-billion pound scheme was being spent efficiently, before simply cancelling the project.
Mr Johnson told reporters: "You have to look at the size of the bill - it's huge.
"You have to consider whether the thick end of £100 billion is being properly spent. You have to consider whether we're profiling that spend correctly.
"I've only been in for 120 days or whatever it is - any new government should look at something of that size if we, as we are, are going to be called upon to make the go/no-go decision.
"But you know where my instincts are. I've overseen a huge amount of very big infrastructure projects, I understand this country's need for big infrastructure and fantastic infrastructure."
He added: "So I'm going to hesitate before simply scrapping something that has been long planned and is of great national importance, but we will want to be checking that the money is being properly spent and that there aren't ways in which it could be reprioritised or reprofiled."
Asked if he wanted to see the bill come down, he replied: "If possible." Phase one of HS2 is planned to run between London and Birmingham.
It was initially planned to launch in 2026, but a recent report by HS2 Ltd stated that this could be pushed back until 2031.
Current designs involve a second Y-shaped phase launching in two stages: Phase 2a from the West Midlands to Crewe followed by Phase 2b from Crewe to Manchester, and Birmingham to Leeds.