Glottal stop insertion at word boundaries is usually not included in descriptions of the phonology of Southern Standard British English (BrE), and little is known about the factors that determine it. This paper investigates the insertion of glottal stops before vowels at word boundaries (e.g. pronounced as [taʊnʔɪz]) in the spontaneous and read speech of 10 speakers of BrE. Results show that glottal stops are inserted in certain contexts in up to 50% of all cases before vowel-initial words at word boundaries. Glottal stop insertion is least frequent after obstruents (11.8%), followed by sonorants (19.0%) and vowels (29.0%). Preceding high vowels and following low vowels also make glottal stop insertion more likely. In addition, glottal stop insertion is slightly more frequent in read than in spontaneous speech, which suggests that it is a feature of the standard rather than a colloquial feature of BrE.