LONDON (Reuters) - Lloyd's of London
"If we do not get access to the EU single market, what will happen is that business will be written by us and others onshore in the EU," John Nelson told the BBC Today programme.
"If we do not see a clear direction of travel, we will have to invoke our contingency plans," he added.
"That would mean moving business, or business leaving London, more quickly than say the renegotiation timetable. So therefore clarity is important, clarity fairly soon."
Lloyd's is the world's leading market for specialist insurance and reinsurance. Some UK-based insurers have already said they were looking to set up regulated subsidiaries in the EU following the referendum vote.
Nelson said around 11 percent of the market's 25 billion pounds ($33.33 billion) in gross written premiums last year came from the European single market.