RoyalBlueNewCorkSethWycombe Royal As far as I am aware it is a rule in the Premier League, but it is NOT a rule the Football League. For us it is just part of the loan agreement.
Obviously I stand to be corrected if that is not the case.......
Luckily nobody on here is actually going to look it up so we can all live happily never knowing.
That's where you are wrong!52.7 During the period of any Temporary Loan Transfer, a Player shall not play against his Transferor Club without the prior written permission of the Transferor Club (which, if given, must be indicated on the appropriate Temporary Loan Transfer form).
Taken from the EFL Official Website. Actually, the bit on transfers is quite interesting, including the money taken by the EFL as part of the transactions.
So it's not against the rules* to play against your parent club, if they give express consent, according to that. I certainly remember plenty of cases (probably a few years ago now) of players playing against their main club... though perhaps more in the cup than League.
*laws