Content and Language Integrated Learning (CLIL) programmes have been incorporated into most European mainstream educational systems since the learning of languages and plurilingualism have received a key role within educational policies. Among the methodological innovations required in effective CLIL implementation, cross-curricular collaboration among teachers remains a pending issue. Nevertheless, recently, some researchers have focused on teacher collaboration in different contexts, since it has proved to constitute a decisive factor in effective bilingual programmes and consequently, it cannot be neglected for longer. In previous studies, cross-curricular collaborative experiences are analysed and the benefits and difficulties found in different contexts are posed. This study aims to prove how collaboration between foreign language (FL) and content teachers previous to CLIL implementation can improve the integration of both areas for the benefit of the students’ use of the foreign language. An intervention proposal was designed and implemented in a Year 5 primary Social Science classroom after analysing the flaws and strengths found in real collaborative practices between the English and CLIL teachers constituting the bilingual staff at the school. The study has revealed how systematic collaboration between both groups of teachers creates richer learning environments where learners benefit, since the use of foreign language and communicative skills are clearly promoted. Nevertheless, we also realise how time-consuming cross-curricular collaboration is and how teachers’ beliefs and opinions differ from one another, even within a small bilingual instructed context, which can hinder effective teacher collaboration.