The campaign to keep Britain in the European Union saw its lead over the rival 'out' campaign fall sharply in the last month, according to a telephone opinion poll by Ipsos MORI.
The survey showed 49 percent would vote to remain in the EU at a June referendum, 5 percent less than in an equivalent poll in February. The poll said 41 percent of respondents would vote to leave, up from 36 percent last month.
Britons will vote on June 23 on whether to remain a member of the EU - the country's most important strategic decision in decades, which will define its future role in global politics and trade.
The poll of 1,023 adults was conducted between March 19 and March 22, meaning it does not fully reflect any impact on voting intentions of bombings in Brussels on March 22 which killed 35 people.
The 10 percentage point reduction in the 'In' campaign's lead is the latest polling evidence to show public opinion is moving in favour of an exit from the 28-country trading bloc.
Ipsos MORI said that when those who were unlikely to vote were excluded from the poll results, the 'In' lead was even narrower.
Britain will hold a referendum by the end of 2017 after Prime Minister David Cameron renegotiates the country’s membership terms.
Cameron is trying to introduce several new reforms as well as to reduce the refugee influx.