The financial research company has questioned the 2 per cent figure that WPP has budgeted for negative growth, stating that the company will "suffer a greater revenue decline" than expected.
It has also warned that Maurice Levy's Publicis may be exposed due to its relationship with General Motors. Melvyn Cooke, an S&P credit analyst, said: "If GM files for Chapter 11, Publicis could face significant losses on its outstanding receivables."