The chief secretary to the Treasury has declined to rule out an emergency no-deal Brexit budget before 31 October, when the UK could crash out of the European Union.

Rishi Sunak refused to confirm whether the chancellor, Sajid Javid, would announce a further stimulus in an emergency budget before a scheduled no-deal departure. The prime minister, Boris Johnson, played down the likelihood of it last week.

On Wednesday, the government announced a £2.1bn funding boost for no-deal Brexit preparations to pay for the stockpiling of medicines, border officials and a major public awareness campaign about likely disruption. It was a move designed to show Brussels the UK is ready and willing to leave the bloc without a deal.

The chancellor usually gives the budget later in November or December. But the Telegraph has reported that Dominic Cummings, Johnson’s senior adviser, told aides to prepare for a budget in early October, just weeks before the scheduled Brexit date.

Paul Johnson, the director of the Institute for Fiscal Studies (IFS) thinktank, has said a pre-Brexit budget would be an “extraordinarily bad idea”, coming before the government could judge what support the economy needed once it had left the EU.

Sunak told BBC Radio 4’s Today programme: “We will have a fiscal event in the autumn because we will need to have a spending review, and a spending review is what sets out all department spending plans for the next financial period.”

'So project fear was in fact reality?': readers on no-deal Brexit funding Read more

When pushed on whether this would take place before Brexit, Sunak said: “We’d like to give people certainty as soon as possible. Obviously we’re working, the chancellor and me, we’ve been in these jobs a few days, we’re working very hard on figuring the exact timing of that. Hopefully we’ll have something to say on that reasonably soon.”

Sunak also defended the huge no-deal Brexit expenditure announced on Wednesday.

“A lot of the money we’re spending is going to go on things we would need to spend anyway because we’re leaving the European Union, that means we’re going to be leaving the single market and the customs union. So of course that does mean changes to how we trade with Europe. So investments in things like our ports and our border infrastructure, our systems, getting companies ready for those changes. All of that is money that we should be spending anyway on those new arrangements,” Sunak said.

Quick guide What Vote Leave leaders really said about no-deal Brexit Show Hide Boris Johnson, prime minister Johnson told the Treasury select committee in March 2016: “Our relationship with the EU is already very well developed. It doesn’t seem to me to be very hard … to do a free trade deal very rapidly indeed.” Speaking at a Vote Leave event in March 2016, Johnson said: “I put it to you, all those who say that there would be barriers to trade with Europe if we were to do a Brexit, do you seriously believe that they would put up tariffs against UK produce of any kind, when they know how much they want to sell us their cake, their champagne, their cheese from France? It is totally and utterly absurd.” Johnson, then foreign secretary, told the House of Commons in July 2017:“There is no plan for no deal because we are going to get a great deal.” Dominic Raab, foreign secretary Two months before the June 2016 referendum vote, Raab told Andrew Neil on BBC Sunday Politics: “We’re very well placed, and mutual self-interest suggests we’d cut a very good deal and it’s certainly not in the European’s interests to erect trade barriers.” During an appearance on the BBC’s Daily Politics in April 2016, Raab added: “The idea that Britain would be apocalyptically off the cliff edge if we left the EU is silly.” Michael Gove, chancellor of the duchy of Lancaster In April 2016, Michael Gove said the UK would have the best of both worlds. “Outside the EU, we would still benefit from the free trade zone which stretches from Iceland to the Russian border,” he said. “But we wouldn’t have all the EU regulations which cost our economy £600m every week.” Liam Fox, former international trade secretary After the referendum, in July 2017, the then-international trade secretary Liam Fox said: “The free trade agreement that we will have to do with the European Union should be one of the easiest in human history. We are already beginning with zero tariffs, and we are already beginning at the point of maximal regulatory equivalence, as it is called. In other words, our rules and our laws are exactly the same.” Simon Murphy and Frances Perraudin

“At some point we will be leaving the European Union, hopefully by the end of October is our clear desires, so at some point we would have to do that campaign to get traders ready for the changes they’ll have to do.”

Sunak added: “Part of this money will be spent on getting them ready, getting them ready for the new trading procedures, getting them ready for what’s called an EORI number so they can fill out the forms they need to do [to move goods in and out of the EU]. Those changes will happen in any case, but we want to accelerate those preparations.”

The shadow chancellor, John McDonnell, described the spending as an “appalling waste” of taxpayers’ money that could have been spent on schools, hospitals and people.