Exchanges, other trading venues and brokers are preparing for huge volumes and high volatility around the Brexit poll. System capacity has been boosted, unnecessary software changes have been put on ice to ensure maximum reliability, holidays cancelled and, in some cases, trading regulations eased to ensure markets don't go into meltdown.

Firms are drawing on the lessons learnt in August 2015, when trading surged due to fears of China's growth, and January 2015, when foreign exchange markets experienced a record day due to policy changes by the Swiss Central Bank.