Global investment bankingrevenues could amount to $204 billion in 2016, down 11% from $228 billion ayear ago, partly due to the U.K.'s vote to leave the EU, according to a study by Boston ConsultingGroup.

BCG revised down its 2016estimate from $212 billion and noted that capital markets are "off to oneof the weakest starts in the last decade," mainly due to a poor firstquarter in both trading and investment banking. The group expects global revenuesfrom primary markets to decline to $40 billion in 2016 from $58 billion theprevious year, with M&A and equity capital markets expected to be the mostseverely pressured.

The group forecasts declines inglobal revenues of 40% to 55% for equity capital markets in Europe, the MiddleEast and Africa, following the Brexit vote. Global revenues for M&A areseen to decline 30% to 60%, and for debt capital markets 15% to 20%.

In fixed income, currencies andcommodities, a decoupling of the U.K. from Europe will send foreign exchangeand rates trading revenues up by an estimated 11% and 8%, respectively, asvolatility in exchange rates and in European government debt drives tradingperformance.

European banks could also requireup to an additional €30 billion to €40 billion in incremental capital in caseBrexit talks lead European banks to convert their bank branches in the U.K.into subsidiaries.

"Overall Brexit will likelyaffect both the short-term revenue outlook, through market shocks and loss ofbusiness confidence, and the long-term revenue outlook through disruptivebusiness transformation," BCG said. "We expect investment banking ROEto continue to fall, and to remain well below 6% in 2016."