Two Nobel laureates and other top scientists are accusing Boris Johnson of destroying Britain’s global reputation by behaving “like a clown” and pursuing a no-deal Brexit that would leave UK science “dead” for years.

The government has assured anxious academics it still has a clear ambition to join the European commission’s new €100bn (£89bn) research funding programme, Horizon Europe, after Brexit. But Robert-Jan Smits, the commission’s former director-general of research, says the UK has “zero chance” of negotiating associate membership after a no-deal divorce.

Top scientists on this side of the Channel warn that billions in funding are at stake, as well as invaluable European research collaborations that cannot be replicated overnight elsewhere.

Sir Paul Nurse, the Nobel prize-winning geneticist and director of the London-based Crick Institute, the biggest biomedical research facility in Europe, says: “Colleagues abroad think the UK has lost its senses. The prime minister behaves like a clown and the world has noted that. Our reputation has plummeted.”

Sir Andre Geim, who won a Nobel prize in 2010 for his work on graphene and is based at Manchester University, is equally damning: “In science, a no-deal Brexit would be like a severe drought for an orchard. You can’t expect to have a harvest after watering it again next year. All the trees are already dead.”

Senior academics say not only future funding is up in the air. They are losing confidence in the government’s pledge, in 2016, to underwrite successful grant applications, submitted before Brexit, to the commission’s current science funding scheme Horizon 2020. Now Universities UK, which represents Britain’s vice-chancellors, is complaining that the cabinet is stalling on the publication of details for universities on how the guarantee would work in practice.

Smits, the commission’s research chief between 2010-2018, says that although it would be legally possible for the UK to join the Horizon Europe programme even after no deal, it would require tough negotiation and could only happen if there was residual goodwill on both sides.

“It all depends on the nature of these last negotiations. If there is a really rough climate and Boris Johnson leaves slamming the door and saying ‘Up yours!’ the chances of an association on science are not close to zero, they are zero,” he says. “At this stage it’s all about trust and, to be frank, some of Boris Johnson’s attitudes and behaviour might not be seen in Brussels as a good guarantee for a decent final deal and for some goodwill remaining intact.”

Paul Nurse, Nobel prize winner and director of the Francis Crick institute: ‘Colleagues abroad think the UK has lost its senses.’

Nurse, one of the commission’s chief scientific advisers, agrees. “I was in Brussels last week and if we crash out as Boris says, I don’t think we stand any chance of joining Horizon Europe.”

He is similarly pessimistic about the short term. “The government has said it will honour any existing commitments under Horizon 2020, but as they have been promising money everywhere I think we have to take that with a pinch of salt. I think they are flying by the seat of their pants.”

If Britain leaves without a deal on 31 October and all funding is frozen, the Crick institute will have nearly £30m outstanding and many more applications left in limbo.

But for Nurse this is about far more than just the cash to carry out essential scientific discovery work. “Europe is probably the biggest science bloc in the world and it is the most effective. The UK is part of collaborations that have been going on for decades, and if we crash out they will be immediately lost.

“Science is a highly international, outward-looking activity. Brexit is driven by the opposite. This is not the way to attract the best brains in the world. We will lose people but it will also become much harder to recruit,” he says.

Prof Geim, meanwhile, says that far from being melodramatic, scientists are underestimating the “dire consequences” of a no-deal Brexit. He says Smits and Nurse are “absolutely right” that the government won’t be able to talk its way into Horizon Europe if Britain crashes out. “Imagine a divorce that ends up in animosity. You don’t expect an invitation from your partner to join a business venture any time soon. That’s the same for no-deal Brexit.”

Geim says some colleagues in the UK have left, and others will leave if there is no deal. He wants to stay – but only if he can keep contributing to society with his research. “If we are deprived of research funding we cannot do what we are trained to do. We become like footballers without a stadium or a ball.”

Gero Miesenböck, an Austrian who is Waynflete professor of physiology at Oxford University, also says many scientists are voting with their feet. “Basic research is the foundation of everything. And the UK is extremely good at it, which is why I am here. But I worry that no one is paying attention to the damage about to be done,” he says.

Prof Imre Berger, chair in biochemistry at Bristol University, who has led numerous European-funded collaborations, and is from Germany, argues that if Britain crashes out, science will be way down negotiators’ list of priorities.

“If this happens the UK will spend years in the scientific wilderness. Science has survived world wars so it will recover eventually, but when we have all this now, what is the point of throwing it away?”

Sir Alan Fersht, a pioneer of protein engineering and one of the most lauded chemists at Cambridge, says: “The EU has been the best thing for British science for decades. The European research council has provided support that didn’t exist, like starter grants for young scientists and advanced grants for senior scientists to do novel work. It has been transformative.”

Vivienne Stern, director of Universities UK International, echoes the alarm. “If we leave without a deal I don’t see any basis for us negotiating associated status on Horizon Europe.” She is worried that the government seems to have no plan B. “It has said nothing on making alternative funding available beyond Horizon 2020. There is no publication date for their review of options, let alone work to design such a scheme – and no funding commitment.”

Meanwhile, universities are anxious that if the tap is suddenly turned off on existing Horizon 2020 grants there is no clarity about how they secure substitute funding to pay research salaries.

“We are really quite frustrated that we are still waiting for details of how the underwrite will work. The Cabinet Office needs to get on with it,” Stern says.

However, asked to comment, a spokesperson for the Department for Business, Energy and Industrial Strategy said the government had provided a funding guarantee that would cover existing successful Horizon 2020 bids.