NEW YORK (AFP) - US stocks finished narrowly lower on Wednesday on the eve of a closely-contested British referendum on whether to leave the European Union.

Wall Street stocks were in positive territory much of the morning, but tilted into the red at midday as two polls put the "Leave" side marginally ahead. Analysts have said a "Leave" outcome could send stock markets tumbling.

"Now we just have to wait for the results," said Chris Low, chief economist at FTN Financial.

The Dow Jones Industrial Average fell 0.3 per cent to 17,780.83.

The broad-based S&P 500 shed 0.2 per cent at 2,085.45, while the tech-rich Nasdaq Composite Index dropped 0.2 per cent to 4,833.32.

Dow members ExxonMobil and Chevron lost 0.4 per cent and 0.9 per cent as oil prices retreated below $50 a barrel following US data that showed a rise in gasoline inventories.

Tesla Motors slumped 10.5 per cent after the electric car maker's US$2.7 billion all-shares bid for SolarCity drew jeers from investors. Analysts questioned the merit of the deal for Tesla, which is led by Elon Musk, chairman of both companies and a large shareholder of SolarCity.

SolarCity rose 3.3 per cent.

FedEx fell 4.5 per cent as it said it was unable to offer a full-year forecast that incorporates its acquisition of TNT Express. Analysts said the market was expecting more detail from the package-delivery giant.

Adobe Systems tumbled 5.7 per cent as it projected third-quarter sales of US$1.42 to US$1.47 billion, implying Adobe may not meet the average analyst forecast for US$1.47 billion.

Dow member McDonald's fell 1.6 per cent following a downgrade by Nomura.