A British think tank says a decision by U.K. voters to leave the European Union would represent a significant shock to the economy — with the pound dropping by as much as 20 percent in the immediate aftermath of the June 23 referendum.

Economists at the National Institute of Economic and Social Research estimated Tuesday that economic growth would slow to 1.9 percent in 2017, compared to a rate of 2.7 percent if Britain remains in the 28-nation bloc.

NIESR joins economists at the International Monetary Fund, the Organization for Economic Co-operation and Development and the U.K. Treasury in warning of the consequences of a British exit.

NIESR says a so-called "Brexit" would see consumer spending per person fall by between 500 pounds and 2,000 pounds a year by 2030.