The UK is on the brink of doing a deal to return to the EU’s £85bn science research programme Horizon Europe. Diplomatic sources say negotiations to become an associate member will continue over the weekend and the two sides are close to agreement after three months of talks, largely over the cost of re-entry. The UK was locked out of Horizon in 2020 in a tit-for-tat dispute over the failure to implement the Northern Ireland protocol trade arrangements in the original Brexit withdrawal agreement. But the door to associate membership was reopened when the replacement Windsor framework was sealed in March, with the European Commission chief, Ursula von der Leyen, declaring a deal could be done “swiftly”. The EU has already said the UK will not be expected to pay for 2021 and 2022, when it was not an associate member, but sources say the UK has been pressing for greater discounts on its contribution to the overall fund. One source said talks had gone backwards in the past few weeks. Under the 2020 Trade and Cooperation Agreement (TCA), UK contributions were to be based on success rates in terms of research funds awarded to UK projects from 2019. The UK argued that contributions should be based on success rates in 2023, which would be much lower given the reduced applications to the fund due to the uncertainty over membership. Data from the European Commission shows a huge drop in awards to British science programmes since 2019. In that year, €959.3m went to the UK in 1,364 grants, compared with €22.18m in 192 grants in 2023 to date. It is understood there was also discussion about what level of under- or over-achievement should trigger a correction and how that would be resolved. Sir Paul Nurse, director of the Francis Crick Institute in London, said a deal would be celebrated by scientists in the UK and across Europe. “I encourage the prime minister to finalise the agreement without further delay,” he said. “Connections with scientists and initiatives in Europe are one of our biggest research strengths and long may they continue.” The foreign secretary, James Cleverly, met the commission’s vice-president Maroš Šefčovič on Monday and the linked membership to Euratom’s nuclear research programme was on the agenda. According to Politico, the UK will recommend that the nuclear programme should be excluded from the deal on the grounds that it is not value for money. Tom Grinyer, the chief executive of the Institute of Physics, said an agreement on Horizon would be “excellent news” for UK science and innovation, but voiced concerns about Britain potentially not rejoining Euratom. “We need to see the final agreement to understand what this means,” he said. A spokesperson for the commission said: “We have no comment to make. As foreseen by the TCA, we are in discussions with the UK on its participation in EU programmes.” A government spokesperson said: “Talks are ongoing and therefore we have not yet agreed a deal.” The government’s plan B involved separate science funds for the UK but science leaders have said membership of Horizon is imperative for international collaboration, talent recruitment and the advancement of science, which they say is not a solo endeavour.