LONDON (Reuters) - Britain's exit from the European Union will have a significant impact on bilateral trade, the European Central Bank's chief economist said on Thursday, warning also against blind optimism as things could turn "nasty quite quickly".

ECB Executive Board member Peter Praet, said: "On Brexit...I think there will be more friction in trade, I am convinced of that," having earlier said the impact on trade in goods and services would be substantial.

He also said it would have a negative impact on investment.

"I remain...optimistic but you cannot be blindly optimistic because these things can turn nasty quite quickly. That is a lesson of history."

(Reporting by Marc Jones; editing by John Geddie)