Effect of Mepolizumab in Severe Bilateral Nasal Polyps

Inclusion Criteria * 18 years of age and older inclusive, at the time of signing the informed consent. * Body weight greater or equal to 40 kilogram (kg). * Male or female participants (with appropriate contraceptive methods) to be eligible for entry into the study. To be eligible for entry into the study, woman of childbearing potential (WOCBP) must commit to consistent and correct use of an acceptable method of birth control from the time of consent, for the duration of the trial, and for 105 days after last study drug administration. * Participants who have had at least one previous surgery in the previous 10 years for the removal of NP. NP Surgery is defined as any procedure involving instruments with resulting incision (cutting open) and removal of polyp tissue from the nasal cavity (polypectomy). For the purpose of inclusion into this study, any procedure involving instrumentation in the nasal cavity resulting in dilatation of the nasal passage such as balloon sinuplasty, insertion of coated stents or direct injection of steroids or other medication without any removal of NP tissue is not accepted. * Participants with bilateral NP as diagnosed by endoscopy or computed tomography (CT) scan. * Presence of at least two of the following symptoms one of which should be either nasal blockage/obstruction/congestion or nasal discharge (anterior/posterior nasal drip) and either nasal discharge (anterior/posterior nasal drip); facial pain/pressure; reduction or loss of smell for at least 12 weeks prior to screening. * Participants with severe NP symptoms defined as an obstruction VAS symptom score of \>5. * Severity consistent with a need for surgery as described by: 1. Participants with an overall VAS symptom score \>7, 2. Participants with an endoscopic bilateral NP score of at least 5 out of a maximum score of 8 (with a minimum score of 2 in each nasal cavity). * Treatment with intranasal corticosteroids (INCS) for at least 8 weeks prior to screening. * Capable of giving signed informed consent Exclusion Criteria * As a result of medical interview, physical examination, or screening investigation, the physician responsible considers the participant unfit for the study. * Cystic fibrosis * Eosinophilic granulomatosis with polyangiitis (also known as churg strauss syndrome), young's, kartagener's or dyskinetic ciliary syndromes. * Antrochoanal polyps * Nasal septal deviation occluding one nostril * Acute sinusitis or upper respiratory track infection (URTI) at screening or in 2 weeks prior to screening * Ongoing rhinitis medicamentosa (rebound or chemical induced rhinitis) * Participants who have had an asthma exacerbation requiring admission to hospital within 4 weeks of Screening. * Participants who have undergone any intranasal and/or sinus surgery (for example polypectomy, balloon dilatation or nasal stent insertion) within 6 months prior Visit 1. * Participants where NP surgery is contraindicated in the opinion of the Investigator. * Participants with a known medical history of human immunodeficiency virus (HIV) infection. * Participants with a known, pre-existing parasitic infestation within 6 months prior to Visit 1. * Participants who are currently receiving, or have received within 3 months (or 5 half lives - whatever is the longest) prior to first mepolizumab dose, chemotherapy, radiotherapy or investigational medications/therapies. * Participants with a history of sensitivity to any of the study medications, or components thereof or a history of drug or other allergy that, in the opinion of the investigator or GSK medical monitor, contraindicates their participation. Aspirin-sensitive participants are acceptable. * Participants with a history of allergic reaction to anti-IL-5 or other monoclonal antibody therapy. * Participants on a waiting list for NP surgery while at screening * Participants that have taken part in previous mepolizumab, reslizumab, dupilumab or benralizumab studies. * Use of systemic corticosteroids (including oral corticosteroids) or corticosteroid nasal solution (intranasal corticosteroid is accepted) within 4 weeks prior to Screening or planned use of such medications during the double-blind period. * INCS dose changes within 1 month prior to screening. * Treatments with biological or immunosuppressive treatment (other than omalizumab) treatment within 5 terminal phase half lives of Visit 1. * Omalizumab treatment in the 130 days prior to Visit 1. * Commencement of leukotriene antagonist treatment less than 30 days prior to Visit 1. * Allergen immunotherapy within the previous 3 months. * Women who are pregnant or lactating or are planning on becoming pregnant during the study. * Participants who currently smoke or have smoked in the last 6 months. * Any participant who is considered unlikely to survive the duration of the study period or has any rapidly progressing disease or immediate life-threatening illness (e.g. cancer). In addition, any participant who has any other condition (e.g. neurological condition) that is likely to affect respiratory function should not be included in the study. * Participants who have known, pre-existing, clinically significant endocrine, autoimmune, cardiovascular, metabolic, neurological, renal, gastrointestinal, hepatic, hematological or any other system abnormalities that are uncontrolled with standard treatment. * Immunocompromized, other than that explained by the use of corticosteroids taken as therapy. * A current malignancy or previous history of cancer in remission for less than 12 months prior to Screening. Participants with successfully treated basal cell carcinoma, squamous cell carcinoma of the skin, or cervical carcinoma in situ, with no evidence of recurrence may participate in the study. * Current active liver or biliary disease (with the exception of gilbert's syndrome or asymptomatic gallstones or otherwise stable chronic liver disease per investigator assessment). * Corrected QT interval (QTc) \>450 milliseconds (msec) or QTc \>480 msec in participants with bundle branch block at visit 1. * A known or suspected history of alcohol or drug abuse within 2 years prior to Screening (Visit 1) that in the opinion of the investigator would prevent the participant from completing the study procedures. * An investigator, sub-investigator, study coordinator, employee of a participating investigator or study site, or immediate family member of the aforementioned that is involved in this study. * In the opinion of the investigator, any participant who is unable to read and/or would not be able to complete a questionnaire.