  We address the issue of anomalous image flux ratios seen in the double-image
gravitational lens JVAS B0218+357. From the multi-frequency observations
presented in a recent study (Mittal et al. 2006) and several previous
observations made by other authors, the anomaly is well-established in that the
image flux-density ratio (A/B) decreases from 3.9 to 2.0 over the observed
frequency range from 15 GHz to 1.65 GHz. In Mittal et al. (2006), the authors
investigated whether an interplay between a frequency-dependent structure of
the background radio-source and a gradient in the relative image-magnification
can explain away the anomaly. Insufficient shifts in the image centroids with
frequency led them to discard the above effect as the cause of the anomaly. In
this paper, we first take this analysis further by evaluating the combined
effect of the background source extension and magnification gradients in the
lens plane in more detail. This is done by making a direct use of the observed
VLBI flux-distributions for each image to estimate the image flux-density
ratios at different frequencies from a lens-model. As a result of this
investigation, this mechanism does not account for the anomaly. Following this,
we analyze the effects of mechanisms which are non-gravitational in nature on
the image flux ratios in B0218+357. These are free-free absorption and
scattering, and are assumed to occur under the hypothesis of a molecular cloud
residing in the lens galaxy along the line-of-sight to image A. We show that
free-free absorption due to an H II region covering the entire structure of
image A at 1.65 GHz can explain the image flux ratio anomaly. We also discuss
whether H II regions with physical parameters as derived from our analysis are
consistent with those observed in Galactic and extragalactic H II regions.
