Before unpacking those materials, here’s a recap of how the case reached this point. 

Who’s suing whom

Foster spent 17 years at GroupM, ending as global CEO of Motion Content Group — the division that co-produced Love Island, managed roughly $500 million in annual entertainment investment, and by his own filed data was posting 140% US revenue growth in his final year. He was fired on July 10, 2025, the day after WPP’s stock dropped 18% on a trading update disclosing serious deterioration at WPP Media. He filed suit in November 2025 against WPP and GroupM (since rebranded WPP Media), seeking at least $100 million, Business Insider reported. 

What Foster alleges

Foster claims GroupM, which according to the complaint controlled roughly $60 billion in annual client ad spend at its peak, ran a hidden profit center by systematically retaining rebates that should have gone back to advertisers. Allegedly, GroupM’s media trading arm would aggregate client budgets to hit volume thresholds with vendors, triggering rebates in the form of free or discounted inventory. Rather than returning those benefits to clients, GroupM allegedly reclassified the inventory as “proprietary media,” sold it back through opt-in agreements, and booked the spread as “non-product related income.” Foster estimates GroupM generated $3 to $4 billion in rebate-driven deals over five years and improperly retained $1.5 to $2 billion of it.

The executives Foster implicates include Mark Patterson, now global president of WPP Media, whom he identifies as the primary architect of the rebate strategy and who publicly called rebates “not a dirty word” in 2016; Andrew Meaden, global chief investment officer, who allegedly institutionalized the practice and in one meeting proposed diverting client spend away from Meta because Meta refused a proprietary deal; and WPP general counsel Nicola McCormick, who Foster alleges privately described the rebate situation as “existential” while declining to formally investigate.

The document at the center of everything

In December 2024, at incoming GroupM CEO Brian Lesser’s request, Foster submitted a 36-page internal report, dubbed “Project Claridges”, laying out both a critique of GroupM’s trading’s practices and a proposal for a new consolidated entertainment division with projected net sales of over $2 billion by 2029. The report contains internal data that is now in the public court record, including a breakdown showing that among GroupM’s top 30 U.S. billing clients, representing $13.5 billion in total billings, only 5% of eligible spend was actually used through the proprietary inventory deals. 

Breaking it down further: among the top 10 clients alone, representing $8.5 billion in billings, 62% of their spend went through non-proprietary channels entirely, and 91.9% of the proprietary inventory generated went unused. Google, GroupM’s single largest US client at $2.3 billion in annual billings, utilized just 0.51% of the proprietary inventory its budget was helping to generate,  meaning 99.5% went unused. These were the clients whose collective spending was being used to hit the volume thresholds that triggered the rebates in the first place.

Lesser acknowledged the report’s concerns and said he’d investigate further. He then asked Foster to send Patterson a “sanitized” version excluding criticism of GroupM’s trading arm. Before Foster could do so, Lesser forwarded the unedited original to Patterson. Patterson, having read a detailed critique of his own practices, told Foster he had “all he needed.” Within hours, Foster’s division was placed under Patterson’s oversight. Six months later he was fired.

Where the money was being spent

The internal documents also show the scale of GroupM’s platform relationships in 2023, establishing the leverage at the center of the alleged scheme. Globally: Google accounted for $9.4 billion in spend, Meta $3.7 billion, TikTok $1.1 billion, Amazon $1.1 billion, and The Trade Desk $1.1 billion. In the US: Google represented $4.9 billion, Meta $1.4 billion, Disney $835 million, NBCU $700 million, Paramount $540 million, and Warner Bros. Discovery $417 million. Total tracked global platform spend: $18.5 billion. U.S. network and platform spend: $9.8 billion.

With that volume to direct, GroupM had considerable power to pressure vendors into proprietary arrangements — and, the complaint alleges, to penalize those who refused.

WPP’s defense

WPP’s motion to dismiss makes three arguments worth taking seriously. 

First and most damaging to Foster: according to a sworn affirmation from Lesser, Foster’s counsel sent WPP a draft complaint on October 10, 2025 — more than two months before filing — and threatened to go public unless GroupM agreed to a large severance payment within 30 days. WPP refused, and the lawsuit followed. WPP argues that offering to stay silent for a payout is fundamentally incompatible with being a whistleblower.

Second, WPP contends that the Project Claridges report — Foster’s supposed evidence of protected disclosure — contains no mention of illegal activity. Its position is that it’s a business proposal for Foster’s own promotion, not a whistleblower document, and that Foster is retroactively reframing an ambitious pitch.

Third, Foster was among hundreds of U.S. GroupM employees and thousands globally let go in a documented restructuring. His entire division was eliminated. Six months elapsed between his last alleged report and his termination.

Why it matters beyond the lawsuit

The China backdrop lends the allegations weight. InOctober 2023, Chinese authorities raided GroupM’s offices and detained more than 30 employees for systematically retaining client rebates —  precisely what Foster claims was happening globally. WPP hit a 27-year stock low in October 2025 when new CEO Cindy Rose publicly conceded WPP Media had “lost its way”. 

Where it likely ends up

A settlement would not be surprising. WPP cannot afford the discovery process, with internal communications about rebate practices potentially feeding both the shareholder litigation and client contract reviews. But the extortion allegation gives WPP real leverage to limit the payout.

The bigger question the case raises has nothing to do with Foster specifically: if GroupM’s own internal data shows its largest clients were almost entirely not benefiting from the proprietary inventory deals that generated nearly $1 billion in annual agency income, what exactly was the business model? That’s a question advertisers, regulators, and shareholders are now all asking — with or without this lawsuit.

WPP declined to comment.