The increasingly tetchy relationship between Sir Martin Sorrell and WPP has entered a new phrase itself as the two parties go to war in a bid for control of Dutch agency MediaMonks.

Sorrell is currently in takeover negotiations for the company via his investment vehicle S4 Capital, in direct competition with the agency network he built, prompting WPP to warn that a £20m payoff comprising share awards is now in jeopardy unless he walks away.

WPP issued the threat by way of its lawyers, Slaughter & May, declaring that any breach of his exit agreement would result in the forfeiture of any outstanding monies owed.

This intervention is unlikely to sway Sorrell however, with one S4 Capital source telling Sky that it amounted to nothing more than "a feeble attempt to destabilise its bid".

Sorrell is determined to start again and build a new multinational marketing services group in the form of S4 Capital, and has amassed a £265m war chest to finance its expansion.

At the time Sorrell stressed that this would be a ‘different type of business’ but has stepped on the toes of his former colleagues in his first significant act.

A non-compete agreement was notably absent from Sorrell’s termination contract, sparking anger among many WPP investors at the potential for future acrimony.