Following the lead from overseas, the S&P/ASX 200 (Index: ^AXJO) (ASX: XJO) is having a bad start to the four-day week with a decline of over 1.5%.

One share that is contributing to these declines is Henderson Group plc (ASX: HGG). It has dropped by almost 7% in early trading on the back of news from the United Kingdom that recent polls are indicating a Brexit is on the cards.

For a long time the June 23 vote on Britain’s European Union membership looked very unlikely to result in an exit. The polls had always shown the remain camp in the lead, with the markets seemingly giving little chance of an exit.

But all that has changed in the blink of an eye. The narrow lead the leave camp carved out last week has now widened into a commanding 19-point lead according to a recent poll.

The Opinium Poll found 52% of respondents will vote to leave the European Union, with just 33% choosing to vote to remain in the European Union. It is worth pointing out that although the poll claims to be unbiased, it was commissioned by the Brexit-backing Bruges Group think tank.

With the markets now believing that a Brexit is a real possibility it comes as little surprise to see Henderson and other companies exposed to the United Kingdom’s financial markets taking a tumble. Other Brexit-related tumbles today include BT Investment Management Ltd (ASX: BTT) dropping nearly 5% and CYBG PLC CDI 1:1 (ASX: CYB) dropping almost 6%.

Many economists believe that fund managers in the United Kingdom are likely see large amounts of funds under management withdrawn as investors seek safety in assets classes such as gold and the Japanese yen.

So while a Brexit is bad news for companies like Henderson and BTIM, it could well be great news for Newcrest Mining Limited (ASX: NCM) and OceanaGold Corporation (ASX: OGC). Personally, I wouldn’t be surprised to see the gold price go beyond US$1,400 in the run up to the vote if it still looks like the leave camp is in control.

Foolish takeaway

For a long time I felt convinced that Britain was going to vote in favour of remaining in the European Union. But right now it is looking more and more likely that a leave vote may prevail. I expect there will be a lot of volatility in the run up to the vote, with the gold miners being best placed to profit.

Finally, if your portfolio has taken a hit related to the Brexit, then I would recommend taking a look at these three fantastic shares which could give your portfolio a big lift.

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Motley Fool contributor James Mickleboro has no position in any stocks mentioned. The Motley Fool Australia has no position in any of the stocks mentioned. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy. This article contains general investment advice only (under AFSL 400691). Authorised by Bruce Jackson.s I contribute to The Motley Fool as a freelance writer and the thoughts and opinions in this post are my own, not that of The Motley Fool’s.