This research investigates the influence of one of the cross-linguistic factors on first language attrition/maintenance of two bilingual groups moved to the UK. This factor is called the regression hypothesis. The bilingual participants in this research are divided into two groups, namely, children bilingual group and adults' bilingual group. The former immigrated to the UK at the ages varying from five to nine, and the latter immigrated to the UK at the ages varying from nineteen to twenty four. This research initiates with defining the cross-linguistic factors. In addition, it concentrates only on the regression hypothesis in the field of L1 attrition in the L2 community. The regression hypothesis is supported by cases studies. In addition, the methodology used in order to gather the required information is fully explained. Finally, it shows the results and it is fully discussed