A Britain Stronger In Europe campaigner (L) stands next to a Vote Leave campaigner outside Parsons Green Tube station in London June 20, 2016. REUTERS/Kevin Coombs

LONDON (Reuters) - Voters in Britain favour staying in the European Union, according to a poll published by social research organisation NatCen on Monday, although most of the interviews were conducted in May.

NatCen said 53 percent of respondents planned to vote to stay in the EU while 47 percent would choose to leave, excluding undecideds, meaning the outcome was “on a knife-edge”.

The survey, which has a sample size of 1,632 respondents, was conducted between May 16 and June 12, before the murder of pro-“Remain” lawmaker Jo Cox last week. Sixty-five percent of the interviews were done between May 16 and 26, NatCen said.

Support for “Remain” was strong in the first fortnight of the field work but waned as the survey wore on, NatCen said.

That pattern was mirrored in other polls but in recent days some polls have shown rising support for “Remain.”

NatCen, which usually conducts social research for governments and charities in areas such as health, said the survey was primarily conducted online. Those who failed to respond were followed up by phone.

Online respondents were more likely to support staying in, at more than 50 percent. Phone respondents were slightly more likely to back a “Leave” vote, the data showed.

“[Our survey] strongly suggests that the truth may well lie in between the two,” said John Curtice, a leading polling expert and a NatCen senior research fellow.

“This implies that, in the final days before the vote, it may well be reasonable to split the difference between the online and phone polls.”