The top Irish executive at US powerhouse Morgan Stanley has said a vote to leave the European Union would be the most significant geopolitical event for the continent since the end of the Second World War.
Morgan Stanley president Colm Kelleher claimed a Brexit could prompt the financial services company to move its European headquarters from London to Dublin or elsewhere.
"This will be the most consequential thing post-war that we've ever seen," Mr Kelleher said in an interview on Bloomberg Television.
"Initially, the fallout could be controlled, but the political ramifications are actually quite profound."
"We are hoping that the British voter will show some sense and listen to all the economic arguments and vote to stay. But we clearly are looking at our plans.
Mr Kelleher (59) joined the company in 1980, and has run its investment bank and trading division since 2013.
He gained oversight of the brokerage last January, when he was promoted to the position of president.
A Brexit vote would cost the UK a number of finance jobs as areas like the clearing of the euro currency would shift to the continent, Mr Kelleher said.
He added that over the longer-term there would be issues related to visas and the movement of labour and goods.
"London cannot not suffer in the event of a Brexit vote, and the reason for that is historic," the Irishman explained. "London has done very well by virtue of being part of the European Union.
"It is (Europe's) market... the exchanges, clearing, everything is based on London."
Even if the UK opts to remain, markets are unlikely to climb significantly or return to higher trading volumes because of prolonged low interest rates and growth, he claimed.
When asked if the US election also affected markets, Mr Kelleher cited oil and China as reasons for investor indecision.
Markets remain on edge heading into today's vote on UK membership in the EU.