A survey of German investor confidence has dipped unexpectedly because of concerns, among others, over whether Britain will vote to leave the European Union.

The ZEW institute said Tuesday its index of economic sentiment for Germany dropped to 6.4 points for May, down from 11.2 in April. Economists had predicted a slight rise to 12.

ZEW President Achim Wambach says positive sentiment from the strong growth of the German economy in the first quarter is being offset by investors' longer-term concerns.

He says "they seem not to expect the economic situation to improve at the same pace going forward; uncertainties regarding developments such as a possible Brexit currently inhibit a more optimistic outlook."

The index is based on a survey of 211 financial experts.