This multi-centre, two-arm, parallel-design, superiority, randomized controlled trial aims to assess the clinical effectiveness of an adolescent-specific strengthening programme compared to usual care for ambulant adolescents aged 12 to 18 years with spastic cerebral palsy (bilateral or unilateral) at Gross Motor Function Classification System (GMFCS) levels I–III. Participants capable of complying with assessment procedures and the exercise programme with or without caregiver support will be individually randomized (1:1) to receive either the strengthening intervention or usual NHS care.