• Patient is 18 years of age or older and who has provided written informed consent.

• Patient has histologically confirmed metastatic MCC.

• Eastern Cooperative Oncology Group (ECOG) performance status of ≤2 .

• Willing and able to comply with all study protocol requirements for the duration of the study.

• Patient must have measurable disease by CT or MRI per RECIST version 1.1 criteria.

• Patient is treatment naïve (no prior systemic therapy for unresectable or metastatic MCC). Note that prior chemotherapy is permitted in the adjuvant setting for loco-regional disease. Prior radiation is permitted for treatment of the primary or loco-regional disease.

• At least 2 weeks since the completion of prior therapy, including surgery or radiotherapy.

• Screening laboratory values, obtained within 14 days prior to registration/randomisation must meet the criteria specified in the protocol.

• Women of childbearing potential (WOCBP) must use appropriate method(s) of contraception

• WOCBP must have a negative serum or urine pregnancy test within within 7 days prior to the start of avelumab treatment and should be performed every 4 weeks in line with other safety bloods or clinical reviews.

• Male patients who are sexually active with a WOCBP must use any contraceptive method with a failure rate of less than 1% per year.

• Patient must be agreeable to have archival tumour material collected