The opinion of Advocate General Athanasios Rantos is not binding on the court, which is examining a complaint from the firm that planned to start the new league, but it will be influential and taken as a sign of the direction the case is taking.
"EU competition rules do not prohibit FIFA, UEFA, their member federations or their national leagues from issuing threats of sanctions against clubs affiliated to those federations when those clubs participate in a project to set up a new competition," he wrote.
The court's final decision on the complaint is not expected until early next year, but the decision will be a relief for the world's biggest football associations and the existing national European leagues wary of allowing some of their richest clubs to break away.
The legal opinion was immediately welcomed by lobby groups representing European football clubs, leagues and supporters.
Fan association Football Supporters Europe said: "Last year, twelve obscenely wealthy clubs tried to destroy European football by creating a closed breakaway league.
"They failed because fans across the continent — including their own —stood in solidarity against their plans."
And the European Club Association hailed Rantos' advice as "a clear rejection of the efforts of a few to undermine the foundations and historical heritage of European football for the many."
The attempt to set up an elite trans-European league in 2021, under the aegis of the European Super League Company (ESLC), initially attracted the support of some of the continent's biggest and richest clubs -- but triggered a backlash from fans and several governments.
The effort fell apart within a frantic 48 hours that threatened to see the European game implode, but left behind bitterness and a key legal question.
The ESLC filed a complaint with a Spanish court accusing UEFA, which regulates European football and organises the Champions League and Europa League, of "abusing its dominant position" in the market to squeeze out fair competition.
The case was appealed to the EU's top court and the eventual ruling will be keenly watched by teams and fans across Europe, worried that a super league would form an exclusive club cornering marketing money and preventing smaller outfits from breaking into the top flight.
If the court follows its advisor's reasoning, the ESLC's challenge will fail.
"Whilst ESLC is free to set up its own independent football competition outside the UEFA and FIFA ecosystem, it cannot however, in parallel with the creation of such a competition, continue to participate in the football competitions organised by FIFA and UEFA without the prior authorisation of those federations," Rantos wrote.
- Closed elite? -
If the court agrees, players from a super league might not be eligible for selection to their national sides in UEFA's European Championship and FIFA's World Cup. Super league teams could be excluded from national leagues and competitions such as England's FA Cup.
European law generally protects commercial competition, but the advocate general found that the soccer governing bodies' threats "may be justified by legitimate objectives related to the specific nature of sport".
In the case of the short-lived super league initiative, this may be a moot point.
Initially it was supposed to create a new competition for 12 rebel clubs: Arsenal, Chelsea, Liverpool, Manchester City, Manchester United, Tottenham, AC Milan, Inter Milan, Juventus, Atletico Madrid, Barcelona and Real Madrid.
Fellow giants Paris Saint-Germain and Bayern Munich refused from the start to take part and the English clubs quickly distanced themselves from the plan in the face of fan fury.
UEFA imposed light fines on the nine clubs that dropped out quickly, extracting a committment that they would not try again, but Real Madrid, Barcelona and Juventus face a disciplinary inquiry that has been put on hold pending the legal ruling.