Holding company did not previously have entire ownership of The & Partnership, which merged with MSix & Partners to form T&Pm.
Account was won following a competitive pitch.
Ahmed founded the business in 1994. It was acquired by WPP in 2012.
Brett Marchand will be interim group chief executive.
The move comes as Roblox is accused of failing to protect children from harmful content.
Campaign asks adland’s brand marketers on the value of individual ad shops.
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