European stock benchmarks made a disappointing start to the month on Wednesday amid falling commodity prices and continued worries about a U.K. retreat from the European Union.

Investors are likely to remain nervous until after tomorrow's meeting at the European Central Bank. While no policy changes are expected after the bank boosted its bond-buying program and slashed interest rates in March, ECB boss Mario Draghi's post-meeting comments will be scrutinized for hints of future action.

The OECD today lifted its forecast for eurozone GDP growth to 1.6% this year from 1.4%, while cutting its projections for the U.K. and the U.S. and warning things could get even worse if Britain leaves the European Union.

The pound was down recently down 0.55% against the dollar, buying $1.44, amid indications from recent polls the "leave" camp is gaining ground ahead of the U.K.'s referendum on June 23 on whether to remain in the EU.

In London, the FTSE 100 fell 0.62% to 6,191.93, with miners Rio Tinto (RIO) and Antofagasta (ANFGY) among the biggest decliners, down 3.8% and 3.1%, respectively, after disappointing manufacturing data from China.

Plumbing distributor Wolseley (WOSYY) was the main loser on London's benchmark index, closing down 5.5%% after warning that "like-for-like" revenue growth had slowed since the end of its third quarter.

In Frankfurt, the Dax dropped 0.57% to 10,204.44, led by power producers E.ON (EONGY) , down 3%, and RWE (RWEOY) , down 2.5%,  as oil prices slipped.

Munich Re closed down 1.1% after announcing 1,800 job cuts at its Ergo unit.

In Paris, the Cac 40 fell 0.67% to 4,475.39

West Texas Intermediate was recently down 0.31% at $48.95 barrel, having pared earlier losses, while copper on Comex was down 0.95%.

Stop and Shop owner Ahold (AHONY) rose 2.5% in Amsterdam after posting first-quarter earnings that Jefferies said featured Ebit that was 5% above consensus forecasts.

But Amsterdam bourse newcomer Philips Lighting faded 1.4% after an underperform recommendation from analysts at that same firm.

In Milan, UniCredit (UNCFF) fell 1.8% as a troubled fundraising at a smaller peer raised concerns about the Italian banking sector and as investors wait to hear who will replace CEO Federico Ghizzoni.