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

The online poll put support for the "Remain" and "Leave" at 44 percent each.

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

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

Britons vote on June 23 on whether to remain in the EU.

(Writing by William Schomberg; Editing by Jon Boyle)