All English language skills require knowledge of words, and the ability to use them appropriately in any given social context. To ensure that learners can successfully participate in communication in English they must gain and develop English lexical competence. Content and language integrated learning approach (hereinafter, CLIL) is recognised as a beneficial method both by policy makers and teachers as it brings added value both to the acquisition of the foreign language and the content of the study subject. CLIL has been one of teaching approaches that can foster the development of this competence. The impact of CLIL on the development of Form 12 students’ English lexical competence in the subject “Politics and Law” was explored in this study. A case study was carried out at a secondary school in Riga, Latvia, from 1 September 2021, until 20 November 2021, with 25 Form 12 students as the research sample. Quantitative and qualitative data
gathered through questionnaires, a pre-test, a post-test, and interviews with CLIL teachers were analysed to assess the impact of CLIL. The results of this study showed that learners’ lexical competence in the target domain improved because of CLIL lessons which both included explicit instruction and created implicit learning opportunities. Explicit teaching was more effective; however, the impact of implicit acquisition might have been lower due to the short duration of the research. CLIL lessons had an overall positive impact on the receptive and productive dimensions of lexicon, as well as its organisation. Receptive dimension was affected more than the productive dimension and the changes in the organisation were less substantial.