The IMF is wading in now, warning of potential 'severe damage' to global economy.
http://news.sky.com/story/1677081/imf-warns-of-severe-economic-damage-of-brexit
This must be what the Leave people say is the 'politics of fear'. I'm not sure why the IMF should want to make this fearful stuff up, but I'm sure someone from Leave will have an explanation.