FXCM ups margins once more against Brexit uncertainty

(Corrects day in paragraph 4 to Wednesday)

LONDON, June 22 (Reuters) - Online currency broker FXCM raised required margins for the second time in less than a month for 18 major FX pairs on Wednesday, advising clients to put aside more funds against possible losses from Thursday's UK referendum on EU membership.

Previously viewed as a sideshow to the wholesale trading between banks and big investment and pension funds that forms the core of the $5 trillion a day global currency market, retail brokers like FXCM (NYSE: FXCM - news) have grown steadily in size and importance.

It and rivals including Denmark's Saxo Bank have all already raised the value of funds they require clients to put aside against currency market bets judged to be exposed to possible big shifts in exchange rates after the UK vote.

The increases, which take effect from 5:00 p.m. Eastern Time (2300 GMT) on Wednesday, relate to a series of dollar, sterling and euro exchange rates against the yen and other currencies.

"We urge all clients to proceed cautiously as market moves may be large and unpredictable," the company said in an email to clients.

"To manage your account risk you may consider depositing additional funds at MyFXCM.com or closing open positions that you have on the affected instruments." (Writing by Patrick Graham, editing by Nigel Stephenson)