Ireland, Cyprus and the Netherlands would suffer most if Britain left the European Union, according to a new study.
The three countries all enjoy strong trade, investment and financial links with the UK, as well as sharing similar attitudes towards trade and regulation.
Other countries identified as being at significant risk from a potential "Brexit" are Greece, Sweden and Germany - but the likes of Romania and Italy could experience very little disruption.
The forecasts appear in a report from the Global Counsel, which examines the possible impact of Britain leaving the EU on countries across the continent.
Prime Minister David Cameron has pledged to hold an in/out referendum on Britain's membership of the EU before the end of 2017.
Ireland is classified as being at "high exposure" from a Brexit due to its strong financial links with the UK as well as its exports to the country, which total almost 12% of GDP including 14.8 billion euro (£10.5 billion) of goods and 5.8 billion euro (£4.1 billion) of services.
Dutch firms have direct investments worth 177 billion euro (£125.8 billion) in the UK, earning more than 9 billion euro (£6.4 billion) in 2013 - equivalent to almost 1.5% of GDP. Cyprus exported 1.3 billion euro (£924 million) to the UK in 2013, and there are extensive financial sector links between the two countries.
These three nations are the only ones named by the report as being at "high exposure" from Brexit, but a total of 14 are listed as being at "significant exposure" and a further six are classed as being at "niche exposure".
This latter category includes France, which the report says is at risk thanks to trade and financial links. French investment in the UK stood at 91.6 billion euro (£65.1 billion) in 2013.
Poland is also in the "niche" category, thanks to high levels of migration with the UK. The report says there are estimated to be 726,000 Poles currently living in Britain, most of whom are young, skilled and economically active. Many send money back to Poland, estimated to amount to 1.1 billion euro (£782 million) in total each year.
Just four countries are listed at being at low exposure: Romania, Italy, Croatia and Slovenia.
"The impact of Brexit on British businesses, the UK economy and wider British interests would be severe and felt across multiple channels," says the report's author, Dr Gregor Irwin.
"The direct impact on the rest of the EU would also be significant. The export, supply chain, investment and policy interests of many large corporates would be adversely affected, but perhaps the single biggest impact will be on the cost of raising finance in Europe which is likely to increase."