The study is planned to randomise approximately 900 subjects (450 subjects per group) with active Crohn's disease who have been diagnosed for at least 4 months, with documented history of disease in the small and/or large intestine, and characterised by a CDAI score between 220 to 450 (inclusive). Subjects will be required to have evidence of current active inflammation by elevated C-reactive protein (greater than the upper limit of normal of the highly sensitive C-reactive protein test) OR an elevated level of faecal calprotectin. Subjects will be allowed to participate in the study while continuing on stable doses of agents typically used to treat Crohn's disease. Inclusion of subjects who received prior treatment with an anti-tumor necrosis factor agent and discontinued due to loss or lack of efficacy will be limited to approximately 50 percent of the study population. Following a 3-week screening period, subjects will be randomised at baseline to receive blinded treatment with one of two doses of GSK1605786A (500 milligrams once daily or twice daily) for 12 weeks. All subjects meeting the definition of responder (CDAI decrease from baseline of at least 100 points) or who are in remission (CDAI score less than 150 points) at Week 12 will be eligible for randomisation into an ongoing maintenance study (CCX114157). Subjects who do not meet the definition of responder or who are not in remission at Week 12 will not be eligible to participate in study CCX114157.