The European Code of Conduct for Research Integrity (jointly published by the European Science Foundation and ALL European Academies (ALLEA), and to which the HKU Policy on Research Integrity makes reference) states the following:
Fairness, in providing proper references and giving due credits to the work of others, in treating colleagues with integrity and honesty. (emphasis added)
It is unacceptable to claim or grant undeserved authorship and to deny deserved authorship, or to inadequately allocate credit.
Unjustified claimed authorship and ghost authorship are forms of falsification.
Authorship should only be based on a creative and significant contribution to the research (i.e. contribution to the design, data collection, data analysis, or reporting, not for general supervision of a research group or editing of text). Guest authorship (i.e. listing authors who do not qualify) or ghost authorship (i.e. omitting individuals who meet authorship criteria) are not acceptable. All authors are fully responsible for the content of the publication, unless it is specified they are responsible only for a specific part of the study and publication.
Sequence of authors should be agreed by all authors, ideally at the start of the project or the initiation of the article/monograph, and may follow national and/or disciplinary codes. The criteria for deciding the order of authors should be agreed at the start of the project or writing.
The work and contribution of collaborators and assistants should be acknowledged if appropriate, with their permission.