The news comes at a delicate time for the network, which is looking to complete a sale over the coming weeks.
Masterfoods this week awarded the Twix brief to the independent Asian roster shop Nitro ahead of Grey US, in a move that is likely to send alarm bells ringing across the network.
Sources close to Masterfoods say Mars is looking to move to creatively led agencies around the world - Grey's reputation has always been based on the quality of its account handling rather than the quality of its creative work.
In the UK, Masterfoods is one of Grey's main clients, alongside Procter & Gamble, GlaxoSmithKline, Dairy Crest and Seat. The agency's relationship with Masterfoods first came under threat when the company appointed TBWA\Worldwide as the third network on its global roster alongside BDDO. In January this year, the appointment was followed by that of TBWA to Grey's $100 million global Pedigree account.
If any more Masterfood brands were to leave Grey, the network's proposed £700 million sale could suffer.
Sir Martin Sorrell, WPP's group chief executive, is the front-runner in the contest to acquire Grey ahead of Havas and various venture capitalists, but as one media analyst said: "Sorrell has always made it clear that the quality of Grey's client list was its biggest attraction."
Grey currently holds the creatve account for 22 Masterfoods brands including the Mars Bar and Galaxy ranges, Twix and Milky Way.