British support for staying in the European Union is up to 42 percent from 40 percent previously while support for leaving is on 40 percent, up from 39 percent, according to a YouGov online opinion poll on Monday.

YouGov said 13 percent did not know how they would vote in the June 23 referendum, down from 16 percent.

Field work for the latest poll was carried out between May 4 and 6 among 3,378 adults and the margin of error was three percent, YouGov said.