Kirk, who joined the financial PR shop from Cazenove as director in June 2003 and left last Friday, said he had quit to spend more time with his fiancee, who is unwell.
Kirk told PRWeek he was taking six months out but would probably not return to an agency position and would instead look for an in-house role or return to corporate broking.
'Mike saw the Square Mile business through a difficult time at the end of the earn out,' said WS European chairman Tim Sutton.
He added that had not appointed a replacement for Kirk and would not rule out scrapping the MD role. But he said WS would 'bring someone in (to WS Square Mile) at a senior level'.
Kirk was promoted to MD of WS Square Mile within five months of joining as part of the shake-up that followed the departure of Square Mile co-founder Tim Jackaman at the end of his earn-out agreement.