The primary objectives of the study are:

• To evaluate the safety and tolerability of three dose levels of MT-1303 in subjects with moderate to severe chronic plaque psoriasis.
• To evaluate the efficacy of three doses levels of MT-1303 in subjects with moderate to severe chronic plaque psoriasis compared to placebo after 16 weeks of treatment on Psoriasis Area and Severity Index (PASI).

Inclusion Criteria:

• Have been diagnosed with plaque psoriasis for at least 6 months prior to screening
• Have moderate to severe chronic plaque psoriasis as defined by PASI score ≥ 12 and BSA ≥ 10% at baseline
• In the investigator's opinion is a candidate for systemic therapy

Exclusion Criteria:

• Non-plaque forms of psoriasis (e.g., guttate, erythrodermic or pustular)
• Current drug-induced or aggravated psoriasis (e.g., a new onset of psoriasis or an exacerbation of psoriasis from beta-blockers, calcium-channel blockers, or lithium carbonate)
• History of any of a list of pre-defined cardiovascular diseases
• History or known presence of any significant infectious, metabolic, oncological, ophthalmological or respiratory system disease or illness likely to render the subject unsuitable for the study.
• Previous exposure to any other S1P receptor modulator
• Receipt of a live vaccine within 28 days prior to randomisation
• Need, or likely need for, treatment with Class I or III anti-arrhythmic drugs or with heart-rate-lowering beta-blockers or calcium-channel blockers, or with any other drugs which can reduce the heart rate
• Clinically significant findings electrocardiogram (ECG) findings.