LONDON - WPP Plc, the world's largest advertising group, said like-for-like revenue in the first four months of the year dropped 6.7 percent after trading worsened in April.

WPP published the key like-for-like metric, which strips out the impact of acquisitions and currency, in a statement released at the start of the company's annual shareholders' meeting on Tuesday.

It said the pattern of trading continued to be as difficult as in the first quarter, although April had been worse.

Chief Executive Martin Sorrell said in April it would not meet its full-year forecasts after first quarter like-for-like sales fell 5.8 percent as companies slashed marketing budgets.

Although revenues were below budget for the first four months of 2009, headline operating profit and headline operating margin were both above budget, the group said on Tuesday.

Profit before tax, mainly as a result of the first-time amortisation of intangible assets in relation to TNS (acquisition), higher interest charges and incremental severance costs, was down significantly on the previous year.

To counter the fall in revenues, headcount fell by almost 4,300 people or 3.7 percent in the period, with over half leaving on a voluntary basis.

Shares in the group were down 2.6 percent at 467 pence at 1157 GMT, underperforming the DJ Stoxx European media sector index which was down 1 percent.

WPP said the economic pressure was most keenly felt in the United States, Britain and continental Europe, although Eastern continental Europe still showed revenue growth for the first four months of 2009.

But the group said despite the tough environment it still took confidence from the fact it could help clients, and make competitive gains, through its geographic spread and experience.

(Reporting by Kate Holton; Editing by Rupert Winchester)