The campaign for Britain to leave the European Union has made up ground to draw level with its rival "remain" campaign ahead of Thursday's EU membership referendum, according to an Opinium poll for the Observer newspaper published Saturday.

The online poll put support for the "remain" and "leave" camps at 44 percent each.

The poll was based on a survey of 2,006 people June 14-17, with most of the work done before news reached respondents of the killing of a British lawmaker Thursday that caused the suspension of campaigning for the referendum.

The previous Opinium/Observer poll, published June 11, showed "remain" ahead of "leave" by 44-42 percent.

Britons will vote Thursday on whether to remain in the EU.