The merger was delayed after European regulators said the paperwork was incomplete. However, on January 24 the European Commission said it did not think the deal would reduce competition significantly.
Grey said the March 7 date was still subject to the requisite approval of its shareholders.
The deal will increase WPP's revenues by 20%. The company, which is headed by chief executive Sir Martin Sorrell, would have had combined revenues of £4.9bn in 2004 had the deal gone through last year.
Ed Meyer, the long-standing chairman and chief executive of Grey, stands to make $350m when the merger is completed.
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