Taxpayers' money will not be spent on preparing for a "no-deal" Brexit until the "very last moment", Chancellor Philip Hammond has suggested.
He told MPs he was preparing for all possible outcomes, including "no deal", and would make money available in a "timely" manner when needed.
But he said he wouldn't take money from other areas, like health or education, now just to "send a message" to the EU.
One ex-minister said billions should be set aside for a "no deal" scenario.
David Jones warned failure to earmark funds for additional customs staff and infrastructure in November's Budget would leave the UK "scrambling" to implement new border controls if there was no deal.
If money was not set aside, he said it would be seen as a "a sign of weakness" by EU leaders who would think the UK was not serious about leaving the EU without a deal.
After the prime minister revealed this week that the government had plans for a Brexit scenario without a trade deal, Mr Hammond stressed the importance of avoiding a no-deal end to negotiations with the EU.
The chancellor, who has been accused of being too pessimistic about Brexit, told the Treasury committee of MPs a "cloud of uncertainty" over the outcome of negotiations was "acting as a dampener" on the economy.
He said this could only be removed by progress in the talks, which he said was dependent on the EU agreeing to discuss its future relationship with the UK as soon as possible.
He told MPs one worst case scenario for a "no deal", would see no air travel taking place between the UK and the EU on Brexit day - 29 March 2019 - but added that he did not see that as likely to happen, even if the UK/EU talks failed to reach agreement.
Writing in the Times ahead of next month's Budget, Mr Hammond said he had a responsibility to be "realistic" about the challenges of leaving the EU and would spend money only when it was "responsible" to do so.
An extra £412m has already been allocated to government departments to prepare for Brexit over the next four years and Treasury sources suggested more would be made available if negotiations faltered.
Asked about the article as he appeared before the Commons Treasury committee, Mr Hammond said he was "committed" to funding departments for Brexit preparation and he was "rather surprised" that the article might be interpreted as saying that he was reluctant to do so.
"We are prepared to spend when we need to spend against the contingency of a no deal outcome," he said.
"I am clear we have to be prepared for a no deal scenario unless and until we have clear evidence that this is not where we will end up."
He said there would be a "decision point" at which departments would have to decide whether to fund programmes after Brexit in the event of no deal being reached but that moment had not yet been reached.
"What I am not prepared to do is allocate funds to departments in advance of the need to spend," he added.
"We should look in each area at the last point that spending can begin to ensure we are ready for a day one no deal scenario. That is when we should start spending hard-earned taxpayers money.
"Every pound we spend on contingency planning on a hard customs border is a pound we can't spend on the NHS, social care or education. I don't believe we should be in the business of making potentially nugatory expenditure until the very last moment when we need to do so."
"We will spend the money in a timely fashion to ensure we are ready but we will not spend it earlier than necessary just to make some demonstration point."
Government sources have sought to play down the significance of Mr Hammond's words, insisting he was merely reflecting Treasury caution ahead of the Budget.
The BBC's assistant political editor Norman Smith said the comments come amid mounting calls from some Tories for extra funds for new IT systems, border posts and staff - likely to be needed if there is no agreement.
Charlie Elphicke, the MP for Dover, said such a course of action would leave the UK "in a great position deal or no deal". "If uncertainty is our enemy, resilience is our friend," he told the chancellor.
On Tuesday, Mrs May - who backed Remain in last year's vote - repeatedly refused to say if she would now vote for Brexit, telling LBC radio: "I don't answer hypothetical questions."
Downing Street sources suggested it was "ridiculous" to say her comments raised doubts about whether she would deliver Brexit, as some critics suggested.