UPDATE 1-Ad group WPP reports 1.6% drop in first-quarter organic growth

(Adds CEO quotes in paragraph 3-4)

LONDON, April 25 (Reuters) - British ad group WPP reported a 1.6% drop in organic growth in the first quarter, lagging its major rivals, after it was hit by loss of business from healthcare client Pfizer and reduced spending by technology companies.

WPP, which had already warned that any organic growth this year would be weighted towards the second half, on Thursday reiterated its guidance for flat to 1% growth this year, with a margin improvement of 20-40 basis points.

Chief Executive Mark Read said the quarter was "very much in line with expectations", and it reflected a tough comparison with the same period a year ago.

"We remain on track to return to growth in the balance of the year, supported by an encouraging new business pipeline and the strength of our business creatively and in media," he said.

(Reporting by Paul Sandle; Editing by Kate Holton and Sarah Young)