Allah Taãla says in the Noble Qurãn:
'Today I have made permissible for you pure things and the food of those who were given the Book (Ahlul-Kitaab) is also Halal for you'. (Qurãn 5:4)
Overtly the above Aayat reads that the Zabihah of the Ahlul-Kitaab is permissible, but the fundamental principle must be understood in order to understand the Qurãn i.e. 'One part of the Qurãn explains the other'. Therefore, this verse should be understood in the light of another verse relating to the same matter: 'Do not eat unless Allah's name has been taken and this (not taking Allah's name) practice is transgression' (Quran 6:121)
While the former verse explicitly states that the Zabihah of the Ahlul-Kitaab is permissible and the latter says, 'Do not eat unless Allah's name has been taken', in the light of both these verses, it is understood that the Zabihah of the Ahlul-kitaab is permissible only if the name of Allah is taken at the time of slaughtering. The Ahlul-kitaab used to take Allah's name when slaughtering an animal, therefore, their Zabihah was also Halal for Muslims. It is for this same reason that an animal slaughtered by the Mushrikeen, etc. is not permissible. The Ahlul-kitaab of today are recognized by name, less faith. There is no guarantee that they take the name of Allah/God when slaughtering an animal. Furthermore, they cannot be anymore trusted in matters pertaining to Halal/Haram.
Since there is uncertainty in the above slaughter manner, the Jurists are unanimous that it is not permissible to consume meat which is doubtful. Unless there is certainty that the Ahlul-kitaab read the 'Tasmiyah' i.e. take the name of God when slaughtering an animal then only will the meat be permissible. We suggest that the Muslims in the U.S.A. and U.K. slaughter the animals themselves. A committee be appointed and look into ways to facilitate for Halaal meat slaughtered by Muslims. This will make them independent from Christian/Jewish sources.
Another proposition may be that the meat sold on the market are packed, sealed and stamped by a Muslim organisation consisting of reliable scholars and Úlama who have proper Islamic knowledge of the principles pertaining to Halaal/Haraam of the Shariáh.
and Allah Ta'ala Knows Best
taken from eat-halal.com