We investigate the formation and the nature of the extended wings of the UV resonance lines of Si IV 1394, 1402A and C IV 1548 and 1550A in the spectrum of the LBV P Cyg. The effects of line blending in the neighbourhood of these lines were estimated by means of a grid of theoretical synthetic spectra, computed for the P Cyg model atmosphere. It is shown that line blending is responsible for the strong wings of these lines with photospheric origin. Our models are next used to treat the problem of the complex structure of [Fe II] lines observed by Stahl et al. in the optical spectra of P Cygni. We find that absorption, emission and P Cygni-type lines blending with the [Fe II] lines are responsible for their complex structure. This offers an example of how line blending can be wrongly interpreted as evidence for blobs in a stellar wind.