The chancellor will press ahead with austerity-busting spending plans, regardless of whether the government secures a Brexit deal in the next few weeks, Downing Street has confirmed.

As Philip Hammond prepares to deliver his budget on Monday afternoon, the prime minister’s official spokesman said “all of the spending commitments that the chancellor will set out today are funded, irrespective of a deal”.

The statement appeared to contrast with Hammond’s suggestion on Sunday that if Britain was forced to leave the EU in March without a negotiated agreement, he would need to “take a different approach”.

Theresa May’s spokesman confirmed the chancellor would be expected to take emergency measures in a no-deal scenario, but stressed that these could be funded by “fiscal firepower” he has set aside – and austerity in public spending would come to an end regardless.

“We have been clear that people need to know that their hard work has paid off and that the austerity that followed the financial crash is coming to an end. That’s why we are focused on delivering, irrespective of Brexit, starting with today’s budget, which locks in all of the progress we have made and puts us on the path to the spending review, where we will set out our long-term approach,” he said.

In an interview with Sky’s Sophy Ridge on Sunday, the chancellor said the proposals being announced on Monday afternoon were based on the assumption that there would be a Brexit deal.

When asked what would happen if there was no deal, he replied: “If we don’t get a deal; if we were to leave the European Union without any deal – and I think that’s an extremely unlikely situation but of course we have to prepare and plan for all eventualities as any prudent government would – if we were to find ourselves in that situation then we would need to take a different approach to the future of Britain’s economy.

“Frankly we’d need to have a new budget that set out a different strategy for the future,” he added.

Labour seized on the apparent contradiction between Hammond’s caution about the Brexit outcome and Number 10’s bullishness about the end of austerity.

A spokesperson said: “Tory budgets are usually plunged into chaos after the chancellor sits down, not before he even stands up. this is confusion on a scale surprising even for this government.”

The health secretary, Matt Hancock, whose department was in line to be the recipient of a significant spending boost, also said it would receive the funding, “whatever the Brexit outcome”.

“The money for the NHS is guaranteed for the long term. It is necessary,” he told Sky News.

Asked about Hammond’s previous warnings, he said: “I think that we are en route to getting a good deal. I also think that if we get a good deal then we’re going to get a boost from that – there’s going to be a deal dividend.

“I talk to companies every day that are ready to make investments in the UK and they want the uncertainty of these negotiations cleared before they will make those investments. So actually I think there’s potential good news on the horizon as we make a deal with the EU.”

However, Hancock declined three times to rule out the possibility of tax rises to pay for extra NHS spending if the economy did suffer a hit from Brexit.

Questioned on BBC Radio 4’s Today programme, he refused to say what might happen if growth was affected. “Clearly, I want to see the economy continuing to grow.”. Pressed again, he said: “Tax is a matter for the chancellor.”

On Sunday, Hammond said the most important spending decision in the budget was the extra cash for the NHS in England announced in the summer, with Scotland, Wales and Northern Ireland getting proportional extra sums in the usual way.

The increase will begin in 2019-20, reaching an extra £20bn in real terms for NHS England by 2023-24. In one of the first indications of how that money will be allocated, MPs will be told that in England at least £2bn will go to mental health services by 2023-24.

Although sizeable compared with the £12bn a year spent on mental health services in England, the Institute for Public Policy Research (IPPR) thinktank said the £2bn extra was only half what was needed to put spending more on a par with that of physical health.

In its 2012 Health and Social Care Act, the coalition government legislated to create “parity of esteem” between mental and physical health, but critics have said this aspiration has never been achieved, and May cited mental health as an area where services must be improved.

According to a government briefing, the new money will ensure mental health support is always available in every large A&E department, the area where people with mental health problems often go because support is not available elsewhere.

The Treasury has also announced that the budget will allocate £60m for spending on tree planting. Most of that will go on a £50m carbon credit programme that will fund an estimated £10m spending on trees over the next 30 years.

This will be Hammond’s third budget and, given the strong possibility of either a substantial reshuffle or a Tory leadership contest within the next 12 months, many MPs suspect it will be his last. It has been prepared amid reports that Hammond and May have clashed over whether or not it is responsible to increase spending and cut taxes in light of the deficit and Brexit.

John McDonnell, the shadow chancellor, said on Sunday that Labour would appeal to other parties in the Commons to join it in voting down the budget if Hammond did not use it to announce a halt to the rollout of universal credit.