"Investors are showing signs of caution ahead of the Fed decision today," said analyst Neil Wilson at trading site Markets.com.

In initial deals, London's benchmark FTSE 100 index of major blue-chip companies was down 0.2 percent at 7,632.69 points compared with Tuesday's close.

The Paris CAC 40 index was flat at 5,511.49 points and Frankfurt's DAX 30 also barely budged at 12,148.42.

European equities had slumped Tuesday on underwhelming corporate results and sour economic data.

The US central bank is expected to cut interest rates on Wednesday but investors are not sure how big the cut will be.