  Context. The CoRoT satellite searches for planets by applying the transit
method, monitoring up to 12 000 stars in the galactic plane for 150 days in
each observation run. This search is contaminated by a large fraction of false
positives, caused by different binary configurations that might be confused
with a transiting planet. Aims. We evaluate the rates and nature of false
positives in the CoRoT exoplanets search and compare our results with
semiempirical predictions. Methods. We consider the detected binary and planet
candidates in the first three extended CoRoT runs, and classify the results of
the follow-up observations completed to verify their planetary nature. We group
the follow-up results into undiluted binaries, diluted binaries, and planets
and compare their abundances with predictions from the literature. Results. 83%
of the initial detections are classified as false positives using only the
CoRoT light-curves, the remaining 17% require follow-up observations. Finally,
12% of the follow-up candidates are planets. The shape of the overall
distribution of the false positive rate follows previous predictions, except
for candidates with transit depths below about 0.4%. For candidates with
transit depths in the range from 0.1 - 0.4%, CoRoT detections are nearly
complete, and this difference from predictions is probably real and dominated
by a lower than expected abundance of diluted eclipsing binaries.
