The head of the International Monetary Fund has cast doubt over claims the NHS would be in line for a cash boost after Brexit. 

Christine Lagarde said she was “hesitant to allocate any Brexit dividend to the NHS” and suggested claims made by Vote Leave that the health service would be handed £350m a week in extra cash were not realistic. 

Speaking alongside Chancellor Philip Hammond in London, the IMF’s managing director said: “It’s an issue of timing.

“You’re comparing apples with oranges. They don’t actually coincide, in timing and in scale.”

In the latest of its regular assessments of the state of Britain’s finances, the IMF also said leaving the EU without a deal would inflict “substantial costs” on the UK economy. 

Lagarde said that all likely Brexit scenarios would “entail costs for the UK economy”, but that a disorderly departure could lead to “a significantly worse outcome”.

While new trade agreements made possible by EU withdrawal “could eventually pare some of these losses for the UK”, any such deals are “unlikely to bring sufficient benefits to offset the costs imposed by leaving the EU”, the international economic think tank said.

And the cost of Brexit will “exceed” any savings from lower contributions to the EU budget.

In its report, the IMF urged the UK authorities to prepare policies to “safeguard macroeconomic and financial stability” in the case of a chaotic Brexit and even if an orderly exit is achieved, with a trade deal covering goods and some services, UK growth can be expected to remain “moderate” over the next few years.

Hammond said the report underlined the need for a Brexit deal to ensure the economic gains of the past decade were not lost.

“We are at a critical juncture for the UK economy. Despite the contingency actions we are taking, leaving without a deal would put at risk the substantial progress the British people have made over the last 10 years,” he said.

Downing Street stopped short of an explicit endorsement of Hammond’s warning.

When asked whether the Prime Minister backed her Chancellor’s assessment, Theresa May’s official spokesman told reporters: “The PM said very clearly that she believes our best days are ahead of us and we will have plans in place to allow us to succeed in all scenarios.”

Lagarde said the UK economy was likely to be weaker under any likely scenario, while a disorderly, no-deal Brexit would have serious consequences.

“It will be a shock to supply. It would inevitably have a series of consequences in terms of reduced growth going forward, increased deficit most likely, depreciation of the currency,” she said.

“It would mean in reasonably short order a reduction in the size of the UK economy.”

The report said the Treasury and Bank of England should be prepared for the possibility of “sharp declines in sterling and other asset prices” and the Bank should be ready to act to ensure the financial system has adequate liquidity.

Plans should also be in place for a possible “significant” increase in interest charged on government debt.

Ministers may have to use the flexibility available to them to boost economic activity by bringing forward major infrastructure projects.

But any easing of fiscal policy – for example by cutting interest rates to stimulate the economy – would have to be temporary and introduced as part of a “credible” longer-term plan to keep debt under control.

The IMF warned that tax rises and spending cuts may be needed if Brexit inflicts lasting harm on the economy, saying: “A permanent shock to output would require an eventual adjustment of revenues or spending.”

Its forecast of around 1.5% growth in GDP for 2018 and 2019 under this scenario was virtually unchanged from its predictions in July.

The UK has already fallen from the top to near the bottom of G7 growth tables in the wake of the 2016 referendum, with business uncertainty holding back investment and the fall in the value of the pound feeding through to higher inflation and slower growth in incomes and consumption, said the report.

The IMF noted that “fundamental questions”, particularly on the Northern Irish border, remain to be answered in order to secure a smooth Brexit.

And it warned: “Resolving these issues is critical to avoid a no-deal Brexit on World Trade Organisation terms that would entail substantial costs for the UK economy – and to a lesser extent the EU economies – particularly if it were to occur in a disorderly fashion.”

The report warned that the UK still faces a “daunting” task to prepare for Brexit and is unlikely to have completed the task by departure day on March 29 2019 even with “the most determined efforts”.

“This risks serious disruptions without an implementation period in place,” it added.

“Co-ordination and co-operation between the EU and UK on priority issues, such as ensuring air traffic continues to flow, would be to the benefit of both parties.” 

 The IMF urged the Bank of England to exercise caution over any further increases in the base rate of interest, following the hike from 0.25% to 0.5% in November last year and to 0.75% in August.

“Further withdrawal of monetary stimulus should await clear confirmation of a durable rise in domestic cost pressures,” said the report.

The organisation said it was “critical” for the UK continue its strategy of “fiscal consolidation” to reduce overall debt levels by keeping a tight hand on the purse-strings.

This meant that the £20bn boost to NHS resources announced by Theresa May in June should be funded from new taxes or cuts elsewhere in the government’s budgets, and not from borrowing, said the report.