Whitmey, left, will join the UK’s biggest supermarket next year to take on the newly-created role of loyalty and membership director, where she will lead on Tesco’s Clubcard loyalty scheme.
Tesco was one of Havas Helia’s (then Evans Hunt Scott) founding clients in 1995 and since then the brand and agency have worked together on Clubcard, which now has more than 16 million members.
When asked whether Whitmey would directly be replaced, a Havas UK spokesman said "various options" were being looked at, including hiring a replacement chief executive.
In a statement, Chris Hirst, chairman of Havas UK and chief executive, Europe, said: "When the opportunity arose to lead the UK’s original loyalty scheme, we couldn’t stand in Tash’s way. While we are hugely disappointed to see her leave, it’s a brilliant opportunity for her and we’re equally excited to have her as a client.
"We’ll be working with Tash and her team over the coming weeks to ensure a smooth handover, as well as the right shape for the Helia business moving forward."
Havas ditched the EHS agency brand in 2015 and introduced Helia as a global network to house its data and digital businesses. Whitmey, who had been the Havas EHS group chief executive since 2011, became Havas Helia chief executive.
Whitmey added: "It’s a real wrench to leave Helia, but I depart in the knowledge that the agency is in great shape, and has an exciting future. The opportunity to work on Clubcard at Tesco was too exciting to miss, and I can’t wait to get started."