This paper argues that centring diphthongs in Received Pronunciation(RP) result from the sequence of a non-low tense vowel and /r/. When a non-low tense vowel is followed by /r/, the sequence is realized as a diphthong which consists of the corresponding lax vowel plus the off-glide [ə]. In terms of Optimality Theory, this can be dealt with by positing a markedness constraint that prohibits non-low tense vowels in the pre-r position. The newly proposed markedness constraint outranks the faithfulness constraints that militate against diphthongization or breaking. The analysis put forward in this paper can account for the occurrence of centring diphthongs whether the following /r/ is in the coda or in the onset.