The EU will be launching their €100bn science scheme, Horizon Europe, in 2021 and the British government has already made clear that they intend to join it. However, Robert-Jan Smits ("the main architect" of the EU's previous scheme, Horizon 2020) has said that a no-deal Brexit would give the UK no chance of becoming a part of it.
Sir Paul Nurse, a British Nobel Prize winner for his work in genetics, agreed with Smits, saying “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.”
Sir Andre Geim, another British Nobel Prize winner, argues that scientists are under-estimating the effects of a no-deal Brexit and that it will be even worse than expected. A lot of his colleagues have already left the UK and many more claim they will also leave if the UK crashes out without a deal.
The UK directly received 15.2% of all Horizon 2020 funding, providing UK projects with around €12.1bn. With the successor to Horizon 2020 having an even larger budget, that's even more money the UK will be potentially losing access to.
Sir Alan Fersht, also, worries about how we will cope without a deal with the EU, saying “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.”