GCI UK and Europe chairman Adrian Wheeler said he was not appointing a replacement for Ryan and would take on her responsibilities 'for the foreseeable future'.
Ryan joined GCI in 1995 as head of its professional and financial services group and became London CEO in 2001.
In the course of her 22 years in the industry Ryan was the first in-house PRO at Arthur Young (now accountancy firm Ernst & Young), and spent six years at Shandwick and three at Hill & Knowlton's Hong Kong office.
'I will take the summer off before my youngest child goes to boarding school in September,' Ryan told PRWeek.
She added that she expected to be in full-time employment before the end of the year.