Eligible participants will be randomised via an independent web-based computerised system to one of two interventions. The research team will provide the relevant contact details/instructions to patients in order to initiate treatment. The trial interventions consist of: (a) The medication sertraline prescribed by their GP according to a trial protocol matching current clinical recommendations and within a dosage between 25 and 150mg daily. We will ask GPs to review patients regularly (at least 6 times in 12 months) and patients to take the medication for a year unless they have significant adverse effects. Side-effects will be regularly monitored. (b) The other intervention is CBT delivered by high intensity therapists from local IAPT services. They will provide 14 to 16 sessions of a manualised treatment developed for use in GAD and will be trained in its delivery. The primary outcome will be the GAD-7 score measured at 12 months. Participants will also be asked to complete this outcome measure by postal questionnaire at 3, 6 and 9 months, as well as a range of secondary outcomes.