Inclusion criteria Informed consent signed and dated Outpatients with moderate to severe spontaneous CU defined by UAS of ≥15 (under the maximum labelled dose of a non-sedating antihistamine Resistant to standard treatment with antihistamines after a minimum of 7 days therapy with the maximum labelled dose of a non-sedating antihistamine (levocetirizine, cetirizine, fexofenadine, desloratadine, loratadine, ebastine, mizolastine Aged more than 18 years Reliable method of contraception for both women of childbearing potential as well as men during the study and 3 months thereafter. A highly effective method of birth control is defined as those which result in a low failure rate (i.e. less than 1% per year) when used consistently and correctly such as implants, injectables, combined oral contraceptives, some IUDs, sexual abstinence or vasectomised partner.