Branded balloons are seen in the office of pro-Brexit group pressure group "Leave.eu" in London, Britain February 12, 2016. REUTERS/Neil Hall

LONDON (Reuters) - The campaign for Britain to remain in the European Union has maintained its lead over those wanting to leave the bloc although the gap has narrowed, according to a telephone poll for Survation published on Thursday.

The survey of 1,006 people, taken before this week’s deadly attacks in Brussels, put the “In” campaign on 46 percent, down two percentage points from the last Survation poll in February, and the “Out” campaign up two points on 35 percent with 19 percent undecided.