Thousands of people take to the streets in a series of 'March for Europe' rallies in protest against the referendum vote to leave the EU

The number of EU nationals working in the U.K. has hit a record high despite concerns of a post-Brexit exodus.

An estimated 2.37 million people from EU member states were employed in Britain between April and June this year, according to the Office for National Statistics (ONS), the highest number since records began two decades ago.

New data published Wednesday showed the number had risen by 126,000 since the same period last year, which preceded but also included Britain's Brexit vote on June 23.

The increase was partly driven by a sudden rise in the number of Bulgarian and Romanian workers within the U.K. Over the past year their numbers have risen by more than a quarter to 337,000 as more workers have taken advantage of the countries' relatively recent access to the U.K. labor market.

Meanwhile, employees from other eastern European countries, such as Poland and Lithuania, which have had access for more than a decade, fell to 997,000 from over a million last year.

The number of workers from 14 long-term EU member states, including France, Italy and Germany, rose slightly to just over a million versus 947,000 in 2016.

The data comes as the British Chamber of Commerce announced Thursday a softening in exporter confidence due to currency fluctuations and a shortage of skilled workers seen post-Brexit. The business trade body said that its 'trade confidence index', which measures quarterly export data, had fallen 2.5 percent since the same period last year.

Statisticians at the ONS noted that while the absolute figure of EU citizens employed within the U.K. had risen over the year, the rate of growth had slowed in the past three quarters.

"The number of workers born elsewhere in the EU continues to increase, but the annual rate of change has slowed markedly," said Matt Hughes, senior labor market statistician at the ONS.

Follow CNBC International on Twitter and Facebook.