In this paper we investigate the security of the two most recent versions of the message authentication code 128-EIA3, which is considered for adoption as a third integrity algorithm in the emerging 3GPP standard LTE. We first present an efficient existential forgery at- tack against the June 2010 version of the algorithm. This attack allows, given any message and the associated MAC value under an unknown integrity key and an initial vector, to predict the MAC value of a related message under the same key and the same initial vector with a success probability 1/2. We then briefly analyse the tweaked version of the al- gorithm that was introduced in January 2011 to circumvent this attack. We give some evidence that while this new version offers a provable re- sistance against similar forgery attacks under the assumption that (key, IV) pairs are never reused by any legitimate sender or receiver, some of its design features limit its resilience against IV reuse.