Romanians carry their luggage for a flight to Heathrow airport in Britain, at Otopeni international airport near Bucharest January 1, 2014. REUTERS/Bogdan Cristel

LONDON (Reuters) - European migrants to Britain receive 10 percent of spending on in-work benefits despite only making up around 6 percent of the workforce, the government said on Monday as it set out why it was backing a deal struck to reform ties with the EU.

Immigration is a top voter concern and Prime Minister David Cameron’s proposals to reduce EU migrants’ access to British welfare payments proved one of the toughest sticking points in his renegotiation of Britain’s relationship with the bloc.

Cameron has argued Britain’s welfare system is a key draw for EU migrants, but critics of his renegotiation say it will do nothing to deter people from coming to Britain as they say most are attracted by job opportunities rather than benefits.

Both the campaign to leave the bloc and those who want Britain to remain in the EU are seeking ammunition to win over the up to 10 million voters who have yet to make up their minds before a June 23 membership referendum.

Cameron has said he believes a UK-EU deal reached last week, which included the right to curb in-work benefits for up to four years, addresses the concerns of British people about tackling the levels of EU migration.

Statistics included in the government report on the deal published on Monday said around 40 percent of recent migrants from the European Economic Area -- the EU’s 28 member states plus Iceland, Liechtenstein and Norway -- were in households supported by the welfare system.

Of the 25 billion pounds Britain spent in 2013-2014 on in-work benefits for workers on low incomes, around 2.5 billion pounds went to EEA migrants, the government said.

“This represents 10 percent of spending, even though people from the EEA make up only around 6 percent of the UK work force,” the government said.

“On average, families with a recent EEA migrant claim almost 6,000 pounds per year in tax credits, and of these, around 8,000 families receive more than 10,000 pounds.”