The second Tosofos on Daf 2a D.H. 'D'oraysa' states that using a possul succah will cause one to come to an issur d'oraysa. This is difficult as surely we are talking about a bitul mitzvas asei at most. I have not seen anyone that speaks about this. Do we find anywhere else where bitul asei is called an issur d'oraysa?
Perhaps we could say that if one makes a birchas hamitzva on this posul succah that one transgresses the lav of li siso and that this is the 'issur d'oraysa.'
Shimon Harris, Manchester, UK
Actively going against a Mitzvas Asei is called transgressing an Issur d'Oraisa, as when the Torah is saying that permanent meals must be eaten in the Sukah, it is obviously also commanding a person not to eat a permanent meal outside of the Sukah. Going against this commandment is going against something that the Torah said not to do., hence the term Issur d'Oraisa. Being "Mevatel an Asei" is a term normally used when one does nothing, such as not taking a Lulav on Sukos and not saying Kriyas Shema, hence being "Mevatel" - "not doing" an Asei.
All the best,
Yaakov Montrose