UPDATE 1-Martin Sorrell quits as head of WPP advertising agency

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LONDON, April 14 (Reuters) - Martin Sorrell quit as the head of WPP on Saturday, leaving the world's biggest advertising agency 33 years after he founded it due to an investigation into personal misconduct.

The British company stunned the industry last week when it said it had appointed lawyers to investigate alleged misconduct by Sorrell, who turned a two-man outfit into the world's biggest advertising group with 200,000 employees.

The 73-year-old denied any misconduct "unreservedly" but in a letter to WPP staff published late on Saturday he said the "current disruption" was "putting too much unnecessary pressure on the business".

He said he had decided that "in your interest, in the interest of our clients, in the interest of all shareowners, both big and small, and in the interest of all our other stakeholders, it is best for me to step aside".

The company said Chairman Roberto Quarta will become executive chairman until a new chief executive is found while Mark Read, a WPP executive, and Andrew Scott, chief operating officer, Europe, have been appointed as joint chief operating officers of WPP. (Reporting by Kate Holton Editing by Kevin Liffey and Alistair Bell)