Major banks have formally told clients to expect volatile currency markets in the aftermath of Tuesday's U.S. presidential election, with the gap between buying and selling prices that determines the cost of trading expected to widen sharply if Donald Trump were to win.

The warnings issued by the electronic trading platforms run by the market's largest player Citi and rivals Barclays and Goldman Sachs, seen by clients of the banks, have over the past two years become standard 'red flags' ahead of big political and economic set-pieces.

But issued broadly for the first time since Britain's vote to leave the European Union in June, they are also a measure of the scale of risks being attached to Tuesday's vote.

Banks have forecast falls of up to 5 percent in the dollar's value against the yen if Trump were to win and a bounce of 1-2 percent for the greenback on a victory for Hillary Clinton.

A number of retail currency trading platforms have also already raised margins clients must hold against open currency bets for fear of sharper moves in the dollar and Mexican peso. One Goldman Sachs client said the bank told clients on Monday it would not accept new stop loss orders on the peso until further notice.

Citi and Goldman Sachs declined to comment.

Barclays said: "Barclays is always available for clients during macro events, providing extra staffing in London, New York and Singapore ensuring 24 hour coverage to monitor markets and handle client trades throughout US election night."

The head of trading with one large London-based investor, a client of Barclays and Citi, said both had issued formal warnings in line with standard procedure since the sudden and dramatic surge in the Swiss franc in January of last year.

"It is very polite and seems like just a box-ticking exercise really: as you are probably aware markets may be more volatile, expect thinner liquidity and wider spreads. It has clearly become the legal norm to do this," the head of trading said.

A second client of several of the larger banks confirmed he had seen such warnings.