About 13,500 financial firms do business into and out of theU.K. under European Union passporting rules, underlining thesignificance of Britain's access to the single market for companieson both sides of the Brexit divide.

Andrew Bailey, chief executive officer of the Financial ConductAuthority, said 5,476 firms authorized in the U.K. use so-calledpassports in EU law to sell services via branches or cross-borderwithout a branch in other countries of the bloc. A total of 8,008firms authorized in other EU states do business in the U.K. underthese rules, Bailey wrote in a letter to Andrew Tyrie, chairman ofParliament's Treasury Committee.

The discrepancy between inbound and outbound numbers goes someway to bolster the U.K. banking lobby's argument that Europeanlenders would be hurt just as much as U.K. firms if Britain losesdirect access to the single market after it secedes from theEU.