Front-month U.S. crude futures were down 53 cents, or 1.1%, at $47.48 a barrel at 0637 GMT, and Brent crude was 49 cents, or 1%, lower at $48.48 a barrel, according to Reuters.
It is noted oil prices have fallen every day after June 8, losing about 8% of their value.
Last week, Brent was at the highest this year, touching almost $53 a barrel, while U.S. crude was near $52 after supply disruptions from producers including Nigeria and Canada, according to the report.
Read alsoTusk: Brexit could threaten western political civilizationWith a week to go before Britain votes on leaving the EU, oil and other markets remain in thrall to opinion polls, which are increasingly showing those supporting an exit are in the majority, Reuters writes.
"The market is going to be soft until next week. The fear is that if the British actually decide to leave the EU there may be some sort of contagion," said Avtar Sandu, senior commodities manager at Phillip Futures in Singapore.
"The rules for exit from the EU are not very clear. There are a lot of question marks over the economic consequences," he said.
U.S. crude stocks also fell last week but the decline was much smaller than anticipated, while gasoline stocks decreased sharply, according to the report.
The U.S. Federal Reserve signaled on Wednesday that it still plans two U.S. rate hikes this year despite slower growth expectations, also hitting the oil market.
