An existing rule means Manchester City and Manchester United will have to quit the Premier League if they enter the controversial European Super League (ESL).

The future of football, both domestically and across the continent, has been thrown into doubt on the back of the much-maligned proposals for a 20-team breakaway - which were confirmed last night.

City and United are two of the initial 12 signatories who have signed up to the ESL, which is set to be bankrolled by US banking giant JP Morgan.

As the brainchild of Real Madrid president Florentino Perez and American owners at three Premier League clubs - including United - the revolutionary midweek competition would come as a direct threat to the Champions League, rather than the domestic system.

However, such has been the widespread backlash against the proposals, the Premier League are among the game's existing governing bodies to have already threatened action against the reform.

Indeed, any clubs and players involved have been told they could be banned from other competitions.

In order for the ESL to be rubber-stamped, though, the clubs involved would require permission from the Premier League to enter - which, on the back of their statement, appears highly unlikely.

Any decision would be judged by the organisation's board and not a vote between the 20 clubs themselves.

Perhaps more importantly, there is already a Premier League rule in place that means City and United - in addition to fellow signatories Arsenal, Chelsea, Liverpool and Tottenham - would have to quit the top division in England in order to follow through with the ESL proposals.

Premier League Rule L9 prevents movements such at the European Super League by stating that clubs are allowed only to enter certain competitions.

In full, the L9 guideline reads: Except with the prior written approval of the board, during the season a club shall not enter or play its senior men's first team in any competition other than:

L.9.1 - The UEFA Champions League;

L.9.2 - The UEFA Europa League;

L.9.3 - The FA Cup;

L.9.4 - The FA Community Shield;

L.9.5 - The Football League Cup; or

L.9.6 - Competitions sanctioned by the County Association of which it is a member.

As suggested, response to the proposed ESL reform - which would see 15 founder clubs then joined by five guests based on domestic success - has been almost universally damning.

Premier League chief Richard Masters has already written to the 20 clubs stating they will not support a reform.

According to Sky Sports, he said: "Premier League rules contain a commitment among clubs to remain within the football pyramid and forbid any clubs from entering competitions beyond those listed in Rule L9, without Premier League board permission. I cannot envisage any scenario where such permission would be granted."

Part of an official Premier League statement read: "The Premier League condemns any proposal that attacks the principles of open competition and sporting merit which are at the heart of the domestic and European football pyramid.

"Fans of any club in England and across Europe can currently dream that their team may climb to the top and play against the best. We believe that the concept of a European Super League would destroy this dream."