Can one reuse positive referee reports if paper ends up being rejected? Context: A submission to a very good generalist journal X received one positive referee report recommending publication and two shorter opinions which both deemed the paper a solid and valuable contribution and thus worthy of publication but perhaps not a priority given the backlog this journal has, thus only weakly recommending publication. Given the fairly high standing of the journal this logically resulted in a rejection.
Questions:

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*Would it be appropriate upon resubmission to a new journal Y to inform the editor of Y of the existing detailed referee report at X? (To be clear: I do not mean sending them the actual report, just making them aware of its existence)

Upshot:the referee's time is not wasted, in particular as the detailed referee report from X is the most thorough report I have ever received (out of over 100), the paper is quite technical in places and the expert referee has spent considerable amount of time on it.
Drawback: The paper starts on the bad footing of being a rejected one, but I think this should not be a big problem if Y is e.g. a specialist journal or if Y is deemed not as "highly ranked" as X. Moreover during the review process at X the referee asked for some clarifications and suggested some improvements which has led to a better paper. I think that even when resubmitting elsewhere we should keep the thank you to an anonymous referee for suggested improvements which already shows the paper was rejected, thus the drawback is there anyway.


*If the answer is yes to 1) should one ask the editor of X beforehand whether it is ok to pass on their contact details to potentially transfer the referee report to the editor at Y?

Personally I think the answer is "Yes" to both questions but hearing other people's perspective is very interesting.
 A: This is not really an answer, but it might be informative.  The BE Journals in economics (of which there are several:  The BE Journal of Macroeconomics, the BE Journal of Economic Analysis and Policy, the BE Journal of Theoretical Economics, etc) label each published paper as belonging to one of four "tiers": a Frontier, an Advance, a Contribution, or a Topic.  Frontiers are supposed to be papers that would be suitable for a very high-end journal, Advances suitable for something a little less selective, and so on.
When you submit a paper, it's considered simultaneously for all four tiers.  The idea is that instead of aiming high and then re-submitting a little lower down after a rejection, one effectively submits to four different journals simultaneously (and there is a single refereeing process where referees recommend not just acceptance, but acceptance to a particular tier).  I'm not sure how successful this has been, but it's been successful enough to survive for, if my memory is correct, about 25 years now.
A: The following might be useful to have in mind:
if your paper is rejected at journal J1 and submitted at J2, the information at the J2 editorial board that it was rejected at J1 might be considered as negative, especially if the editorial board of J2 considers J1 as being of equal or lower level (which might not coincide with your own appreciation, or of J1's editorial board or referee in case they recommended J2 as alternative — keep in mind that the appreciation of the journal level varies according to subtopics and countries).
A: For many journals the referee is asked to tick a box when they submit their report to indicate whether or not they (1) allow the report to be used for another journal and (2) whether their identity may be disclosed to the editors of that other journal.
Given this practice, the answer to question 1 would be a "yes". Whether or not the report is then actually transferred from journal X to journal Y (anonymously or with the identity of the referee disclosed) would be something journal X decides based on whether or not the referee authorized them to do so.
Concerning question 2, I don't think you need permission beforehand from journal X  when you inform journal Y of the existence of a detailed report.

This complication will hopefully become a thing of the past, when more and more journals migrate to a practice where referee reports are public documents, even if anonymous.
