Three and Wieden & Kennedy have parted ways after 10 years as the mobile network reviews its advertising account.
It follows a major restructure at Three in March that merged the customer and marketing teams, leading to the departure of chief marketing officer Shadi Halliwell.
AAR is assisting Three with the process, which is now at chemistry stage.
The ad account is the latest in a string of reviews that the brand has been carrying out over the past year, including its media and CRM businesses, which were won by Zenith and MRM McCann respectively.
A spokesman for W&K said: "Over the past 10 years, we’ve had an incredibly successful partnership with Three, creating industry-leading work which has helped double their customer base and market share.
"So it’s with regret that given the changes the brand is going through, our partnership has had to come to an end after a brilliant decade together. It’s a great brand, so we wish all of the agencies participating the best of luck in the review. We hope you have as much fun over the years to come as we’ve had working on it."
W&K’s most recent work for Three, "Real 5G", illustrated a future world when 5G has changed things for the better and earned a Campaign Pick of the Week. However, the press and social iterations of the campaign were banned for being misleading by the Advertising Standards Authority.
"#HolidaySpam" was the most-awarded campaign for effectiveness in the UK in 2015, according to the Warc Effective 100. The work also won gold at the 2016 UK Effie Awards.
A Three spokesman added: "As part of best business practices, we continuously review our agency partners. We do not comment on specific processes."