One in ten technology startups have lost out on investment since the vote for Brexit last year.
Founders said that potential investors had held off from a funding round, or withdrawn it completely, based on a survey of more than 200 tech leaders by Tech London Advocates.
Just over 15 per cent said they have had customers withdrawing from deals due to uncertainty in the UK.
And more than half believe the status of London as a global tech hub will be damaged once Article 50, the official signal that Britain will leave Europe, is invoked.
This will happen in less than a week's time as Prime Minister Theresa May will trigger it on 29 March, kickstarting trade negotiations with countries across the region.
Just under a quarter are confident that it will have little impact on the capital's world leading tech reputation.
A third also said they had experienced employees raising concerns about future rules on visas and immigration regulation.
“The survey data shows that many tech leaders continue to feel uneasy about the process of leaving the EU – with both investment and access to talent under threat," said Tech London Advocates' founder Russ Shaw.
And one founder spoke of the impact directly on his business, losing out on staff from abroad .
“At Metail, we have seen the fallout from the decision to leave the European Union firsthand," said its founder and chief executive Tom Adeyoola.
Prospective employees have opted to move to other European firms, or have even chosen to stay abroad rather than return to the UK – with one current staff member deciding to accelerate taking a professorship because he felt that research funding would be restricted once Britain leaves the EU."
He added: "The government must work to mitigate the damage from leaving the European Union to startups across the UK, guaranteeing access to international workers, research funding and investment to protect the UK tech industry.”