The chancellor said he had a responsibility to be “realistic” about the challenges of leaving the EU.
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.
He said he would be “prepared for all outcomes, including a no-deal scenario” but added that the best stimulus for the economy was “certainty”.
This – he wrote – could be achieved by “reaching a deal on the terms of our future long-term relationship with the European Union“.
An extra £412m has already been allocated to government departments to prepare for Brexit over the next four years.
She told LBC radio: “I don’t answer hypothetical questions.”
The PM added: “I voted Remain for good reasons at the time but circumstances move on.”
Presenter Iain Dale asked Mrs May why she could not say she had changed her mind, given that she was leading the country into Brexit.
Pressed again, Mrs May said: “I am being open and honest with you. What I did last time round was I looked at everything and I came to a judgement and I would do exactly the same this time round.
“But we are not having another referendum and that’s absolutely crucial.”
Downing Street sources suggested it was “ridiculous” to say her comments raised doubts about whether she would deliver Brexit, as some critics suggested.
Mrs May’s second in command, First Secretary of State Damian Green, was asked the same question.
He told Newsnight “it would have been” better had the country voted Remain.
Liberal Democrat deputy leader Jo Swinson said: “It is staggering that even the prime minister isn’t convinced by the government’s approach to Brexit.”
Former UKIP leader Nigel Farage tweeted: “How can Theresa May negotiate Brexit without believing in it?”