Update (6/24/16 10:16 a.m. EST): Britain has voted to leave the E.U. Shortly after results were announced (the"leave" campaign won by 52 percent), Prime Minister David Cameron said he would be stepping down.​ 

On Thursday, June 23, British citizens will vote to decide whether or not the U.K. will remain in the European Union. 

Former mayor of London Boris Johnson and nearly half of Conservative members of Parliament are just some of those in favor of a British exit (a.k.a.  Brexit). They say the nation would be stronger on its own, claiming that British influence abroad has diminished over the past four decades due to the changes within the E.U.

Meanwhile, those against an exit--like Prime Minister David Cameron, President Obama, and a majority of economists--are leading the polls (though some suggest pro-leave sentiment is growing). They warn that leaving the E.U. would be a costly move for Britain. 

Others foresee a simultaneous dip in the stock market, bond market, and the pound sterling if Brexit takes place. The British pound has already sunk in value this year amid uncertainty leading up to voting day. Regardless, a "leave" in the referendum result would not actually take effect until at least 2018, as exiting would require two years' notice.

In the meantime, here's how Brexit may affect the U.S. economy.

Implications for the U.S. economy.

Much of the economic impact would depend on the trade deals made between the U.S. and U.K.--and the terms they carry. 

Currently, President Obama is negotiating an overarching free trade agreement with the E.U., called the Transatlantic Trade and Investment Partnership. It is set to conclude in the next few years, but Obama has indicated he has little interest in a deal with an independent Britain. 

A free trade partnership is no trivial matter for American companies, who exported $492 billion worth of goods to Europe in 2015 (including $56 billion to the U.K.), according to the European Commission.

Concerns from U.S. businesses.

American companies that set up operations in Britain as a gateway to the European market are likely to be impacted the most. "The biggest single concern is that the U.K. would not be able to replicate the kinds of trade deals it currently enjoys with third parties," said Geoffrey Heal, a professor of social enterprise at Columbia Business School. He warns that Brexit could lead to increased tariffs.

ZeroUV, an eyewear boutique based in Huntington Beach, California, recently made a bigger push for the U.K. market by selling its sunglasses through Amazon Prime U.K. Brian Fujita, a co-founder of the Inc. 5000 honoree, says about 5 percent of the company's global online traffic (60,000 unique viewers) comes from that market.  

He believes Brexit would increase regulations on U.K. transactions for global companies. "We're still a small company, so we're not quite sure how we would prepare for that, or how to update our model," said Fujita.

Thierry Soudee is the founder and CEO of UpClear, another Inc. 5000 honoree with an office in Europe. He thinks Brexit is highly unlikely, but even with higher expenses, his company would benefit from maintaining a presence in a "business-friendly" location.

How U.K. businesses are preparing.

Mehdi Jaffer, head of marketing at AlphaSights, an information services firm, appreciates the rich cultural diversity his hometown of London has to offer. "We don't know if Brexit would actually affect that, but it seems like a step in the wrong direction in terms of integration," he said.

Since launching 2008, the London-based company has grown to 300 employees--half of whom are based in the U.K., and many of whom are immigrants. An exit could pose hiring challenges by making it harder for foreign workers to secure British work visas.

AlphaSights has expanded to new markets in New York City, Hong Kong, and Dubai. Although Jaffer doesn't anticipate operational challenges in the event of a British exit, he's glad the company is already working to set up an office in Germany. AlphaSights grew its sales by 680 percent between 2011 and 2014, and landed a spot on the 2015 Inc. 5000 Europe list (at No. 155). "If we do leave the E.U., we'd still have a foothold in Europe's biggest economy," he said.

Neal Slateford, co-founder of Bath, U.K.-based Lovehoney, agrees with Jaffer. He believes the ability to hire multilingual customer service staff has been key to growing his business. 

Launched in 2001, the sex toy retailer now pulls in nearly $80 million (£55 million) in annual sales--about $10 million of which Slateford attributes to the 50 Shades of Grey film franchise. "We find it very easy to grow our business by recruiting people from Europe -- French, English and German speakers," Slateford told Inc.  

Still, he points out that a direct-to-consumer business like Lovehoney wouldn't be directly impacted--at least not its sales. "There might be a short shock in terms of currency, but I think it would only be temporary," he said.