Bankes leaves the CFO role with immediate effect, but will remain at the group until 1 November to assist with the completion of the merger.
Porta chairman John Foley said: "On behalf of the Board, I would like to take this opportunity to thank Rhydian for his many significant contributions to the group since joining in January 2016. He has been a valued member of the team and leaves us in a strong position as we embark on this next chapter in our story as SEC Newgate S.p.A. We wish him every success in the future."
Porta said a further announcement will be made in due course regarding the role of group chief financial officer.
London-listed Porta and Italian holding company SEC announced plans to merge in June, in a deal that values Porta - which owns PR shops Newgate, Publicasity and Engage Comms - at around £4.5m.
The plan is to float the combined operation on AIM.
The merger is set to create a business with global fee income of around €80m, with offices in the UK, Europe, the Middle East, APAC and South America, and a workforce of more than 550.
Porta is not commenting on what the merger means for individual agencies at this point.
SEC – which acquired a 19.3 per cent stake in Porta for £3m in 2017 – includes London public affairs specialist Newington, plus agencies in Italy, Spain, Belgium and Germany.