London’s standing as Europe’s leading destination for tech start-ups is at risk if the British Government does not clarify how it plans to keep the best technical talent, entrepreneurs and investors have warned.

In an open letter to Prime Minister Theresa May, nine leading UK-based technology entrepreneurs and investors, including Skype co-founder Niklas Zennstrom, pressed the government to act to ensure a continued flow of skilled migrants after Britain leaves the European Union. It also called on the government to address EU market access and other issues.

“The No. 1 concern for entrepreneurs post-Brexit is access to talent, in particular technical talent,” they said in the letter. It was dated Dec 6 and timed to coincide with the annual TechCrunch Disrupt London conference for start-ups and investors. “Quotas on specific skills could severely limit the ability of new tech companies to grow,” they said.

Signatories of the letter also included Balderton Capital partner Bernard Liautaud, the now London-based founder of French software firm Business Objects, as well as Brent Hoberman of Founders Forum and Sonali De Rycker, partner at Accel Partners.

The letter calls on the government to campaign for access to the European Union’s digital single market and to ensure a simple and competitive framework for companies, labour, tax, stock options and bankruptcy