Mayor of London Sadiq Khan has said the government must not delay in reaching a deal to re-join the EU’s research and innovation programme.
Horizon Europe was set up to help sponsor projects across Europe.
Several leading scientists have said that being outside of the €95bn scheme was highly damaging to UK science and innovation.
The government said constructive talks were ongoing and no deal has been agreed on.
The UK left the scheme in January 2020 following the Brexit referendum and talks on re-joining it began in March.
A previous assumption was that if differences over the Northern Ireland Protocol could be resolved, the UK would fully re-join the Horizon programme under terms similar to those it had before.
But BBC News understands that Mr Sunak is interested in an alternative research programme put together by ministers, known as “Pioneer”.
This would be a UK-led programme involving collaboration with non-EU as well as European nations.
It was developed in case a research agreement could not be reached with the EU.
Sources say that while some aspects of the Horizon programme are appealing to the prime minister, such as grants to individual scientists, he believes that larger institutional grants favour France and Germany and may not represent good value for money.
‘Lost out on benefits’
However, Mr Khan said it was “crucial” for London as a “home to some of the world’s leading scientific institutions” that there was no delay in re-joining the programme.
“Our withdrawal is yet another example of London being damaged by our departure from the EU, and means we have lost out on huge benefits to the capital including access to funding and collaborations on projects,” he said.
“The government must not delay its decision to re-join the scheme if we are to restore our role as a leader in European science.”
Martin Rees, a prominent scientific figure and former president of the Royal Society in London, told the BBC: “We cannot waste any more time – New Zealand are now on board and we should be too.
“Sunak may think he’s securing greater value via protracted negotiations, but prolonging the delay leads to further missed opportunities and will make it harder for UK science to restore its standing and its collaborations.”