A candidate in the Welsh Labour leadership contest claims he has been the subject of a "stitch-up".
But Mr Miles alleges the union later declared him to be ineligible for its nomination.
Unite - who nominated Mr Gething for the Labour leadership - insists its nomination process was conducted properly and according to its rules.
After the hustings, a discussion was held by the political committee to nominate the union's preferred candidate.
Mr Miles says the committee was told he was ineligible to be nominated due to a rule "that no one was aware of".
"This new rule was not declared when I was invited by Unite to attend and participate in the nomination hustings," he said.
"Neither was it referred to during the hustings. After the hustings concluded, the leadership of Unite failed to mention it to me. To date, I have not seen any evidence the rule even exists for the purpose claimed."
Mr Miles said he did not want to make a statement on the matter but that he was "standing up for the principle of fairness and equal treatment".
The decision has been described by Sky News sources as a "stitch up".
A spokesperson for Unite said Mr Miles was interviewed and had his "suitability considered" during a meeting of the Unite Wales Labour Party Liaison Committee.
"However, Unite's rules conference last year decided that Unite 'will only formally endorse candidates who have held elected lay office as representatives of workers'. Under this rule he was ineligible for nomination.
"Unite is satisfied that the nomination process was carried out correctly."
A spokesperson for Vaughan Gething's campaign said: "We're proud and honoured to welcome Unite's nomination for Vaughan Gething for Welsh Labour leader and first minister of Wales."