With less than two weeks to go until U.K.'s referendum on whether it should stay in the European Union, fear of a "Brexit" have been looming over global markets.

Asian and American markets have been down on the possibility that the U.K. could leave the EU. But why? Not only will an exit have a profound impact on uncertainty levels globally, it may force U.S. companies to reconsider their international business models.

Many U.S. companies headquarter their European operations in the U.K. Without a unified regulatory framework companies will have to weigh up whether the U.K. is still a viable hub for their European businesses, adding unexpected costs.

JP Morgan's (JPM - Get Report) Jamie Dimon this month warned if the U.K. were to leave the EU he would "have no choice but to reorganize our business model here."

He added, "Brexit would mean fewer JP Morgan jobs in the U.K. and more jobs in Europe." JP Morgan has 16,000 employees in the U.K.

And Dimon's plan is looking more and more likely.

Polls released last night conducted by YouGov for The Times put "leave" 7 percentage points ahead of "remain."

The Sun newspaper urged its readers to vote to "leave" the European Union in the June 23 referendum. The tabloid's front page today said, "BeLEAVE in Britain." The Rupert Murdoch-backed Sun is the most widely read newspaper in Britain and is the first to voice its view on the upcoming referendum.

This endorsement has seemingly stunned business leaders into silence. While no leader has come forward for the "remain" camp today, BT's (BT CEO Gavin Patterson yesterday warned employees that an exit could have significant impact on the business.

The referendum is also weighing on central bankers' decisions. The Federal Reserve, Bank of England and Bank of Japan are scheduled to have meetings this week. It is unlikely that rates will be changed by the Fed or the Bank of England but the prospect of a leave will certainly be considered in the meetings.

After last month's meeting, Bank of England Governor Mark Carney gave a stark warning, saying that an exit would force the U.K. back into recession.

This has weighed heavily on European markets. The FTSE 100 was down 1.23% in midday trading. In Frankfurt, the Dax was down 0.78% and the Cac 40 was down 1.59% in Paris.

In perhaps another blow to the "remain" campaign the Union of European Football Associations soccer league has threatened to kick England out of the Euro cup competition currently taking place in France after violence among supporters.

Over the weekend England supporters were involved in skirmishes with Russian fans ahead of both teams' kickoff game of the tournament. England next plays fellow home nation Wales on Thursday. UEFA has warned that if there is violence ahead of the game it would put an end to England's tournament.

In a soccer-mad nation, that would likely turn sentiment further against the EU.

All eyes are expected to be on the U.K. for the next week, as the campaign looms over global markets.