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 a healthcare client 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. (Reporting by Paul Sandle; Editing by Kate Holton)