LONDON (Reuters) - The chairman of the world's biggest advertising group WPP said the company had not disclosed more about its investigation into former CEO Martin Sorrell's personal misconduct because he resigned before that became necessary.

Sorrell, the driving force behind 33 years of dealmaking and expansion at WPP, stepped down this month after the board investigated an allegation of misconduct.

WPP did not give any details of the alleged misconduct and Sorrell denied the charges.

Executive Chairman Roberto Quarta told a first-quarter results conference for analysts on Monday that Sorrell quit after the investigation had concluded but before the board had considered its implications.

"Martin decided to resign before the board had taken into consideration the outcome of the investigation and determined whether or not it was appropriate to take action," he said.

"The matter...was really what we consider to be a matter of privacy and therefore is a matter for Martin and hence the reason why we did not disclose," Quarta said.

(Reporting by Sarah Young and Kate Holton; editing by Paul Sandle and Jason Neely)