See also: 

Elsevier's preference is for authors to resolve such matters amongst themselves, although that is not always possible

Complainant must be made aware that the matter cannot be investigated unless the journal Editor informs the corresponding author or author about whom a complaint has been made (as a matter of “due process”) and possibly the institution or company at which the research took place (the complainant may not wish to make the complaint at such a formal level)

In that communication (see   Form Letter A), the Editor should indicate that the matter may be referred to institution or company where research took place or any other relevant institution or agency (for example a funding agency) unless the author provides a reasonable explanation (accepted as reasonable by Editor) (NOTE: some agencies such as the NIH’s Office of Research Integrity, will not consider disputes that are solely about authorship)

If corresponding (or complained-about) author accepts the position of the complainant...

...then publication of a correction, corrigendum or retraction procedures are the normal remedies (note that there may still be disagreement concerning the appropriate classification of the complainant’s contribution to the paper or how the complainant is identified)

If corresponding (or complained-about) author rejects the position of the complainant...

...then the Editor will have to consider whether the author’s explanation is reasonable—normally the Editor would also inform the complainant of the author’s explanation and seek comment (see   Form Letter B)

If the corresponding/complained-about author has not responded to the Editor’s correspondence...

...then the Editor may want to refer the matter to institution or company at which the research took place (see   Form Letter C)