Britain risks losing its leading position in drug research and development if plans for regulating medicines after Brexit are not set out soon, one of the UK's most influential research leaders has warned.
Prof Jeremy Farrar, head of the Wellcome Trust, called for "clarity".
He said delays could slow down the development of drugs.
Ministers have said they want to maintain a close relationship with the European medicines regulator.
Prof Farrar was speaking at a meeting in London, organised by the body that regulates medicines in the UK, the MHRA.
He told BBC News that large pharmaceutical companies were waiting to see details of the government's plans for regulating medicines after Brexit.
"If that clarity is not forthcoming and people can't be sure of their long-term stability and the clarity of those future relationships, then inevitably that will be factored into their investment decisions.
"That's inevitable. That can be avoided," Prof Farrar said.
In a letter to the Financial Times, Health Secretary Jeremy Hunt and Business Secretary Greg Clark said that their aim was "to ensure that patients in the UK and across the EU continue to be able to access the best and most innovative medicines and be assured that their safety is protected through the strongest regulatory framework and sharing of data".
But more than three months on from that joint statement, Mr Farrar would like to see some action from the ministers.
"I've no doubt but it's high on their agenda," he said.
"What is needed is clarity around where the negotiations are going soon so that people who are planning their investments today for 2021, 2022 and 2025, which is what boardrooms are currently doing, get it as quickly as is possible."
The UK has led the way in developing a European regulatory system that is regarded by researchers and industry as having the perfect balance between ensuring patient safety and encouraging innovation.
The NHS and the country's excellence in medical research have encouraged many of the UK's pharmaceutical companies to have large research and development operations in Britain and conduct clinical trials here.
This allows patients to benefit from new drugs quickly.
And being part of the European Medicines Agency, whose regulations are closely linked to the US and Japanese medicines regulators, means that new drugs developed in companies based in Britain have something close to a one-stop regulatory shop to have access to a vast international market.
As a result, the costs of medicines can be kept down.
Prof Farrar is concerned that the UK would become isolated if it failed to reach an agreement with the EU over close medicines regulation after Brexit.
He said: "We are in a golden age of the development of medicine. If you look around the world to those countries which are relatively isolated in terms of just a national regulatory agency, as opposed to being part of a wider regulatory environment, the uptake of that innovation does tend to be slower.
"All negotiations are complex but if people are willing to compromise where necessary and people are willing to ensure that ultimately the [wellbeing] of European Union citizens and those of this country in a post-Brexit world are at the heart of those negotiations, then I think it's doable."