User blog comment:It's Good to be the Queen/Gwen's Revenge - a Pendragon heir/@comment-4892425-20121112123602/@comment-5674726-20121112163449

...citing Camelot's inheritance and succession laws, legally declares her illegitimate issue; therefore ineligible for the throne.

Except that, however Camelot's succession laws are structured, Morgana's illegitimacy is apparently not a barrier to her having a claim to the throne.

We could dismiss Morgause, Agravaine and Morgana herself as having ulterior motives to believe that she has a claim to the throne but that doesn't explain Gaius, who has no reason to want to see Morgana on the throne, flat-out stating that Morgana's royal blood means that, if Uther dies, only Arthur stands between her and the throne. It also doesn't explain why Uther's reason not to acknowledge Morgana was for Arthur's sake. Nobody would think less of Uther for fathering a child outside wedlock, and it certainly wouldn't be any kind of negative reflection on Arthur. The only way that Morgana's paternity being known would have a negative impact on Arthur is if it gives Morgana a claim to the throne.

I would say that the most probable scenario, based on the indications given on the show, is that Morgana would have a claim to a place in the line of succession but only after Arthur and any children he has. Since there are no other Pendragon claimants apart from Uther's two children, it's possible that Morgana is in a stronger position in terms of her claim to the throne than she would be if she and Arthur has Pendragon uncles and cousins.

In Season Three, Morgana and Morgause generally planned around the idea of getting Arthur out of the way so that Morgana would be heir to the throne rather than expecting that they could press Morgana's claim ahead of Arthur's stronger one. They must have known that, even if she could make a case for being the spare to the throne, Arthur was the heir and, as long as he is even semi-competent as a ruler, a sister would have minimal support, if any, as a rival claimant. It's not until they have their immortal army at their disposal that they make a move to seize the throne.

Of course, given her attempts to seize the throne, it's likely that she is considered to have forfeited any succession rights she would usually have so, while she might have been heiress presumptive after Arthur became King had she not tried to seize the throne and found a way to make Uther reveal her paternity, she doesn't hold that position now.