  We report the discovery of WTS-1b, the first extrasolar planet found by the
WFCAM Transit Survey, which began observations at the 3.8-m United Kingdom
Infrared Telescope. Light curves comprising almost 1200 epochs with a
photometric precision of better than 1 per cent to J=16 were constructed for
60000 stars and searched for periodic transit signals. For one of the most
promising transiting candidates, high-resolution spectra taken at the
Hobby-Eberly Telescope allowed us to estimate the spectroscopic parameters of
the host star, a late-F main sequence dwarf (V=16.13) with possibly slightly
subsolar metallicity, and to measure its radial velocity variations. The
combined analysis of the light curves and spectroscopic data resulted in an
orbital period of the substellar companion of 3.35 days, a planetary mass of
4.01+-0.35 Mj and a planetary radius of 1.49+-0.17 Rj. WTS-1b has one of the
largest radius anomalies among the known hot Jupiters in the mass range 3-5 Mj.
