INCLUSION CRITERIA:

1. Participant must be ≥ 18 years at the time of screening.
2. Histologically- or cytologically-documented NSCLC and have been treated with concurrent CRT for locally advanced, unresectable (Stage III) disease
3. Provision of a tumour tissue sample obtained prior to CRT
4. Documented tumour PD-L1 status ≥ 1% by central lab
5. Documented EGFR and ALK wild-type status (local or central).
6. Patients must not have progressed following definitive, platinum-based, concurrent chemoradiotherapy
7. Participants must have received at least 2 cycles of platinum-based chemotherapy concurrent with radiation therapy
8. Participants must have received a total dose of radiation of 60 Gy ±10% (54 Gy to 66 Gy) as part of the chemoradiation therapy, to be randomised. Radiation therapy should be administered by intensity modulated RT (preferred) or 3D-conforming technique.
9. WHO performance status of 0 or 1 at randomization
10. Adequate organ and marrow function

EXCLUSION CRITERIA:

1. History of another primary malignancy, except for:

   • Malignancies treated with curative intent and adequate follow-up with no known active disease and have not required active treatment within the past 3 years before the first dose of study intervention and of low potential risk of recurrence.
   • Adequately resected non melanoma skin cancer or lentigo maligna without evidence of disease .
   • Adequately treated carcinoma in situ, including Ta tumors without evidence of disease.

2. Mixed small cell and non-small cell lung cancer histology.

3. Participants who receive sequential (not inclusive of induction) chemoradiation therapy for locally advanced (Stage III) unresectable NSCLC.

4. Participants with locally advanced (Stage III) unresectable NSCLC who have progressed during platinum-based cCRT.

5. Any unresolved toxicity CTCAE >Grade 2 from the prior chemoradiation therapy (excluding alopecia).

6. Participants with ≥ grade 2 pneumonitis from prior chemoradiation therapy.

7. History of idiopathic pulmonary fibrosis, drug-induced pneumonitis, or idiopathic pneumonitis - regardless of time of onset prior to randomisation. Evidence of active non-CRT induced pneumonitis (≥ Grade 2), active pneumonia, active ILD, active or recently treated pleural effusion, or current pulmonary fibrosis

8. Active or prior documented autoimmune or inflammatory disorders (with exceptions)

9. Active EBV infection, or known or suspected chronic active EBV infection at screening

10. Current or prior use of immunosuppressive medication within 14 days before the first dose of durvalumab.