it is a kabirah but not kufr - al `iyadhu billah. the basic principle of ahlu's sunnah across the board among all madh'habs without any difference is that committing a kabirah - an enormity - does not make one a disbeliever. as long as one does not deny that it is forbidden. for example, if a muslim drinks wine, he is committing a kabirah; but if he does not believe that it is forbidden or believes that it is permissible in islam, he goes out of the pale of islam. one of the tenets of the khawarij creed was that committing a kabirah makes one a disbeliever. therefore, every sunni book of aqidah contains this clarification. Allah ta'ala knows best.
I have heard different answers. In one speech Shaykh Hamza Yusuf (a man I greatly admire, perhaps my favourite modern scholar!) said that 'it is a kabair [enormity/mortal sin] but it is not kufr' but other people have told me that it takes you out of Islam. I prefer Shaykh Hamza's view since it is more logical but I'd like the opinions of the Hanafi jurists please.
