NEW YORK (Bloomberg) -- Crude fell a fourth day after closing at a one-week low as concern mounts over the strength of global growth as Britain votes later this month on whether to leave the European Union.

Futures fell as much as 0.8% in New York after settling at the lowest since June 3 on Monday as investors looked ahead to a June 23 referendum that will determine Britain’s membership in the EU. Shale drillers added 12 oil rigs in the U.S. over the past two weeks in the first consecutive weekly gains since August, according to Baker Hughes Inc. data.

Oil has surged about 85% from a 12-year low in February as the global glut is trimmed by disruptions and a slide in U.S. output, which is under pressure from the Organization of Petroleum Exporting Countries’ policy of pumping without limits.

West Texas Intermediate for July delivery fell as much 38 cents to $48.50/bbl on the New York Mercantile Exchange and was at $48.53 at 7:44 a.m. Tokyo time. Total volume traded was 49% below the 100-day average. The contract slipped 19 cents to settle at $48.88 on Monday.

Brent for August settlement dropped 19 cents to end the session at $50.35 on the London-based ICE Futures Europe exchange on Monday. The global benchmark crude closed at an 83-cent premium to WTI for August delivery.