Net migration to Britain increased by more than a third in the past year, official figures showed Thursday, making a key pledge by Prime Minister David Cameron to cut numbers even more improbable.

The difference between migrants leaving and arriving in Britain rose to 243,000 in the year to March, from 175,000 the previous year, the Office for National Statistics (ONS) said.

Cameron wants to reduce net migration to below 100,000 per year before Britain's next general election on May 7, 2015.

Immigration is expected to be one of the key political battlegrounds at the election, at which Cameron's center-right Conservatives fear a strong challenge on the right from the eurosceptic UK Independence Party (UKIP).

The Conservatives -- in a coalition government with the center-left Liberal Democrats -- have brought in curbs on new workers and students wanting to come to Britain in a bid to address the issue.

Two-thirds of all immigrants to Britain during the year -- 214,000 out of 560,000 -- came from within the European Union.

Immigration from Romania and Bulgaria more than doubled to 28,000 after labor market restrictions for their citizens were lifted at the start of 2014.

Immigration Minister James Brokenshire hit the airwaves soon after the figures came out to insist that Cameron's administration was doing enough to limit immigration.

"The government has reformed benefits, healthcare and housing rules to make them among the tightest in Europe and we continue to see an increase in the number of British citizens in work," he said.