Poorta's appointment is seen as crucial to maintaining the merged agency's relationship with COI Communications. He has played a pivotal role in D'Arcy's relationship with COI - D'Arcy handles more than £33 million in COI advertising a year.
Forestier-Walker, who had worked at Burnett on a part-time basis, is leaving to set up her own consultancy. The chief executive of Burnett, Bruce Haines, said that the agency would continue to use her for project work.
"Miranda has done an outstanding job and we are extremely pleased to be able to offer her support in her new future," Haines said.
Despite the appointment of Poorta, Mark Stockdale, the executive director of strategic consultancy, will continue to run Leo IQ. Poorta is expected to take up his new position at the end of the year.
Further changes are expected at the agency when details on the structure of the new organisation are finalised.
It is still not clear what will happen to other members of D'Arcy's senior management team, including Nick Hastings, the creative director of D'Arcy, and Luca Lindner, the president and chief executive of D'Arcy for Europe, the Middle East and Africa.
Insiders say Hastings is unlikely to be among the staff moving to Burnett's Sloane Street office. Lindner's close ties to Leo Burnett and the D'Arcy client Fiat, which is reviewing its pan-European agency suppliers, mean Burnett is likely to find him a role.
Poorta's decision to join Burnett comes little more than a week after the D'Arcy managing director, Barry Cook, accepted the new position as chairman, reporting to Haines.