UEFA president Aleksander Ceferin has said that clubs should not be allowed to host European Cup matches in their stadium in the future if visiting fans are not permitted to attend.
Ceferin told German broadcasters ZDF that he was siding with Bundesliga club Eintracht Frankfurt whose fans have been denied attending Wednesday's last-16 clash at Italy's Napoli by prefecture of Naples decree.
Under UEFA rules, hosting clubs must make available 5% of their tickets for visiting fans, and Ceferin said they would now update the rules accordingly.
"We must say that if something like that happens the match won't be played there. It is simple: we must change the rules," Ceferin said.
"It is not acceptable that the Italian authorities decide that German fans are not allowed. This situation is intolerable. We must urgently do something about it because the decision of the authorities is absolutely not correct."
Frankfurt meanwhile acknowledged that some their supporters would still be in and around Napoli's stadium for the game.
After much to-ing and fro-ing, Naples authorities stopped the sale of tickets to fans from Frankfurt due to security concerns after trouble at last month's first leg, won 2-0 by the Italian league leaders.
But Dario Minden from the Frankfurt fan department thinks the ban will be counterproductive.
"The safest thing is always to have people in a visitors' block. Whether you think it's good or bad, realistically Naples will not be a Frankfurt-free zone despite all efforts," he told dpa on Tuesday.
"The situation has become more dangerous due to the supposed security measures."
Germany's interior and sports minister Nancy Faeser said she had "little understanding" for the decision to exclude Frankfurt fans.
"In the case of high-risk matches, every possible security measure should be examined very carefully before excluding all fans of a team as the very last option," she told dpa. "Such a drastic measure may not contribute to de-escalation."
Frankfurt's lawyers won an appeal against an original decision by Italian authorities to bar all German residents, with Ceferin saying UEFA was involved.
But Naples authorities reimposed a ban specifically blocking ticket sales to citizens of the city of Frankfurt.
Without evidence, German supporters have accused Italian police of trying to ease Napoli's passage to the quarter-finals by removing the famously passionate Frankfurt fans.
They helped carry their team to Europa League glory last season, famously taking 30,000 to Barcelona despite only having an official ticket allocation of 5,000 in the last-eight clash.
Theoretically, Eagles fans who live outside Frankfurt can get tickets for the Napoli game but those who bought them already and reside in Frankfurt will struggle to get their money back on flights and accommodation, according to an expert.
Tourism lawyer Paul Degott told dpa: "It's a difficult and complex situation, but I think the Eintracht fans have been unlucky."
German sports lawyer Thomas Summerer meanwhile says the steps taken by the authorities in Italy are not proportionate.
"Restrictive measures to maintain public safety and order must adhere to a strict framework. In view of the trouble in the first leg, there may be a risk of problems in the second leg. But I do not consider this merely abstract danger sufficient to justify a blanket ticket ban," he said.
"The ticket ban on fans, who live in Frankfurt, is against European law."
But Germany's police union (GdP) said the exclusion of fans from a certain city at football matches is "conceivable" in Germany too.
"It should also be feasible if the police have the necessary information. However, such a measure can and should only be a very special exception," GdP federal chairman Jochen Kopelke told dpa.
