Near infrared photometry of IRAS sources with colours like planetary nebulae. III.

We present the near infrared photometry of a new sample of 225 IRAS sources, many of them previously unidentified in the literature, selected because their far infrared colours are similar to those shown by known planetary nebulae. The results obtained are used to establish the main source of near infrared emission. Combining this information with the far infrared IRAS data and a few additional criteria we determine the nature and evolutionary stage of all the sources observed so far, including those for which near infrared photometry was previously reported in Papers I and II. Among the unidentified IRAS sources in our sample we find only a small percentage of planetary nebulae, many of them very young and dusty, showing peculiar near infrared colours. Most of the new objects observed in the near infrared are identified as transition objects in the previous stages of the stellar evolution. Among them, we find heavily obscured late-AGB stars, early post-AGB stars still obscured by thick circumstellar envelopes which are probably the true progenitors of planetary nebulae, and a significant fraction of stars with bright optical counterparts showing little or no near infrared excess, which we associate with highly evolved post-AGB stars with low mass progenitors, which may never become planetary nebulae. In addition, we also find a small percentage of young stellar objects, as well as a few Seyfert galaxies. We conclude that, in most cases, based on near infrared data alone, it is not possible to give a confident classification of the unidentified IRAS source. However, the near infrared is shown to be a powerful tool, specially when dealing with objects which are heavily obscured in the optical. In this case, the detection of the near infrared counterpart is the only way in which we can extend the study of these sources to other spectral ranges and may be crucial to understand the short-lived phase which precedes the formation of a new planetary nebula