** HK shares of HSBC Holdings 0005.HK slip 2.1% to HK$85.3; set for biggest one-day pct fall since April 7

** Stock falls to lowest since April 29; top pct loser on Hang Seng Finance Index .HSNF and index tracking financials stocks .HSCIF, which are down 0.4% and 0.3% respectively

** HSBC HSBA.L Chairman Mark Tucker, the bank's first-ever externally-recruited chairman, will retire by year-end after nearly eight years in the role

** Co says Tucker will continue as a strategic adviser to CEO Georges Elhedery

** Adds it has already begun looking for the next chairman

** YTD, HK stock gains 12.9%, London-listed shares up 5%

(Reporting by Donny Kwok)

((donny.kwok@thomsonreuters.com))