LONDON (Reuters) - Net migration to Britain fell to its lowest level in more than two-and-a-half-years in 2016, official data showed on Thursday, driven in part by an increase in the number of European Union citizens leaving the country.

The Office for National Statistics (ONS) said net migration to Britain in the year to December 2016 was 248,000, down 84,000 on the previous year and the lowest estimated level since the year to March 2014.

Concerns about immigration played a big role in Britain’s vote in last June’s referendum to leave the EU. The government has repeatedly failed to meet a pledge to reduce the net annual level to below 100,000.

Prime Minister Theresa May’s ruling Conservatives, who are expected to win a June 8 national election, have repeated that promise, and have also said Britain must be able to control the number of people who come from the EU after Brexit.

The ONS said last year’s drop in net migration was driven by both a statistically significant rise in the number of people leaving Britain, mainly EU citizens, and a non-statistically significant fall in the number of people arriving in Britain.

Net migration of EU citizens fell 51,000 year-on-year to 133,000, it said, driven by a decrease in the number of citizens from the so-called EU8 countries -- the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Slovakia and Slovenia.

The ONS said net migration for those eight countries fell by 41,000 to 5,000, the smallest estimate recorded since they joined the EU in 2004.