The founder of fintech firm Transferwise said if it was starting again it would not set up in Britain - because of Brexit uncertainty.
The comments from Taavet Hinrikus, the firm's chief executive, were a blow as the Government announced measures to help the UK "lead the world" in the sector.
Mr Hinrikus also said the money transfer company is to set up a European headquarters in order to retain access to the single market.
Transferwise, founded in 2011 in London, employs about 120 people in the capital and handles more than £1bn in transfers every month.
Mr Hinrikus said that after Brexit its global headquarters would remain in the UK, which is its biggest market.
But he added: "Uncertainty means that maybe if you're building the next fintech business you shouldn't build it in London today, until everything clears up again and we understand what's going to happen with access to talent and so on."
Mr Hinrikus said the company had yet to decide where its new European headquarters will be but it will be set up by the Brexit process deadline of March 2019.
It will mean jobs in Europe that would otherwise have been in Britain, he said.
He said the main reason for the move was the likely loss of "passporting" rights which allow EU financial services firms to sell their services freely across the bloc.
Mr Hinrikus made the remarks while appearing at a Government-sponsored event backing the fintech, or financial technology, sector in London - where Chancellor Philip Hammond and Bank of England Mark Carney were key speakers.
He said: "We have to invest in the skills of the future and our economy must remain at the cutting edge - not just of fintech, but of AI (artificial intelligence), biotech - of every area in which we have the potential to lead the world into this new industrial revolution."