  The probability of W-boson decay into a lepton and a neutrino in a strong
electromagnetic field is calculated. On the basis of the method for deriving
exact solutions to relativistic wave equations for charged particles, an exact
analytic expression is obtained for the partial W-decay width at an arbitrary
value of the external field strength. It is found that, in the region of
comparatively weak fields, field-induced corrections to the standard decay
width of the W-boson in a vacuum are about a few percent. In these conditions
at first we observe the decrease of the W-boson partial decay width with the
increase of the external field strength parameter. At absolute minimum the
W-width deviates from the corresponding vacuum value by a factor 0,926. Then
with further augmentation of the background field intensity the W-boson decay
width grows monotonously. In superstrong fields the partial W-width is greater
than the corresponding one in vacuum in a dozen of times.
