Stretton will lead Proximity’s creative department of more than 40 people until the end of the year, when a permanent successor to Ryan is expected to be in place.
The four creative partners at Proximity will report to Stretton, who became the non-executive chairman at AIS in April. Stretton has stepped down from this role and will report to Proximity’s chief executive, Mike Dodds.
Dodds said: "There is no-one I would trust as much to manage and inspire our creative department during this interim period."
Dodds added he was "close to a decision" on a permanent executive creative director.