Market players also eyed the release of U.S. employment data on Friday, amid ongoing uncertainty over the future of the Federal Reserve's stimulus program.
On the Comex division of the New York Mercantile Exchange, copper futures for September delivery traded at USD3.153 a pound during European morning trade, down 0.7% on the day.
New York-traded copper prices fell by as much as 0.9% earlier in the session to hit a daily low of USD3.147 a pound.
The ECB was expected to leave interest rates on hold on Thursday and to reiterate that an exit from loose monetary policy remains distant.
Concerns over political instability in Portugal continued to weigh on market sentiment following the resignation of country’s foreign minister on Tuesday and the finance minister on Monday in protest over government austerity policies.
The political crisis raised doubts over the future of the country's coalition government and its ability to honor bailout commitments.
Copper prices rallied to a two-week high of USD3.178 a pound on Wednesday, after data showed that the U.S. private sector added 188,000 jobs in June, more than expectations for an increase of 160,000.
Market players now looked ahead to Friday’s highly-anticipated U.S. nonfarm payrolls data for indications of how the recovery in the U.S. labor market is progressing.
Any improvement in the U.S. economy was likely to reinforce the view that the Federal Reserve will begin to taper its bond purchase program in the coming months.
The Fed’s stimulus program is viewed by many investors as a key driver in boosting the price of commodities as it tends to depress the value of the dollar.
Elsewhere on the Comex, gold for August delivery dipped 0.2% to trade at USD1,250.15 a troy ounce, while silver for September delivery eased down 0.2% to trade at USD19.58 a troy ounce.