HS2 costs would make it "very difficult" to scrap first phase of project
Sir Amyas Morse, the outgoing chief of the National Audit Office (NAO), said the country would have to be in "a lot of economic trouble" to pull the plug on the mammoth project.
There have been calls from some MPs to cancel the planned high-speed railway, which has a £55.7 billion budget, and invest the cash in infrastructure projects elsewhere.
Speaking to the BBC's Westminster Hour, Sir Aymas suggested the first phase would have to be completed due to the cost and work already incurred.
He said: "There is a point where there's not much point thinking you can go back. Are we at that point on HS2? It's difficult for me to say.
"It might be that we still haven't quite crossed the Rubicon on it, but pretty soon we'll have sunk so much in buying land, building track and so forth, that it would be very difficult not to at least go to Birmingham."
He added: "I think that to pull HS2 now, we'd have to be in a lot of economic trouble."
-> 'HS2 is a load of rubbish': Councillors stick knife into high speed rail projectThe NAO has investigated the project's acquisition of land on the London to West Midlands section after concerns were raised over how much it was estimated to cost.
The watchdog said estimates of the cost of land and property had "increased significantly over time", although such estimates are inherently hard to get right.
Nevertheless, some MPs have been highly critical of the cost of the project.
Last month Andrew Bridgen, Tory MP for North West Leicestershire, labelled it a "white elephant that grows ever larger on huge amounts of taxpayers' cash".
Former minister Dame Cheryl Gillan had asked an urgent question in the Commons, saying it was "almost impossible to hold this monster to account".
Former Cabinet minister David Davis also called for the project to be cancelled and the funds spent elsewhere on the rail network.
The New Economics Foundation think tank also warned that the project would widen the north-south divide by drawing passengers towards the capital.