Jenny Biggam, who co-founded the7stars in 2005, said the management team wanted to set up a new sister company because the existing agency has billings of £260 million and 150 staff and they do not want to jeopardize its "unique culture" by becoming too big and unwieldy.
Biggam claimed there is growing demand from clients "for a new breed of media agency" that is "independent from the major holding companies".
She maintained the new company will be more than "a conflict shop" to handle rival clients and suggested it will work "in beta" by using data, content and technology in innovative ways that could be adopted later by the7stars.
The new agency is yet to be named. It will have separate branding, its own management team and be based in a different office from the7stars’ central London headquarters in Aldwych.
It is expected that the7stars and the new agency will be owned jointly through a newly-created, mini-holding group structure.
Daglish will join at the end of the year after quitting Arena Media. He said: "There is a huge opportunity for a new independent agency in the UK. From a personal perspective, the opportunity to create a brand new business, starting with a blank sheet of paper, is a great creative challenge. It is a once-in-a-lifetime job offer."
The7stars is one of the most successful independent media agencies in the UK, after increasing billings by 80% last year when it was twice named Media Agency of the Year – at the 2015 Media Week Awards and in the 2015 Campaign Annual.
Biggam and her co-founders, Mark Jarvis and Gareth Jones, have positioned the7stars as an alternative to the big ad groups because the agency plans and buys media on a client-by-client basis and insists it does not do volume and share deals based on all its clients’ total spend.
Daglish worked with Biggam at Carat earlier in his career and joined Arena in 2006 when it was still an independent, called BLM.
She said: "The launch of a new independent media agency will provide a much-needed alternative to brands and advertisers while helping the7stars to continue its award-winning work, independent agency values and unique market approach. Henry is the perfect choice to run it, and I can’t wait for him to get started."
Havas, the parent company of Arena and the world’s sixth biggest ad group, bought BLM in 2008. Daglish has been managing director since 2013.
Paul Frampton, chief executive of Havas Media Group UK & Ireland said: "Havas Media Group has a clear investment and succession plan in place for its leadership team. We are currently in the process of seeking a new CEO for Arena Media, which we will now accelerate.
"Elliot Parkus, who is Henry’s number two, will assume leadership of Arena in the meantime. After ten years at the helm, Henry has made an amazing contribution to Arena Media and is leaving it in great shape. We will miss him but wish him every success for the future."