Study to Test the Effect of CDP870 in the Treatment of Crohn's Disease Over 26 Weeks, Comparing CDP870 to a Dummy Drug (Placebo), Following 3 Doses of Active Drug (CDP870).

Inclusion Criteria: * Definitive diagnosis of Crohn's disease confirmed (at least 3 months prior to study entry) either by radiological, endoscopic or histological evidence, affecting the terminal ileum (L1), colon (L2) or ileocolon (L3)\*. \* Vienna Classification (1998) * Active Crohn's disease (≥ 220 and ≤ 450) scored over the 7 days prior to the first dose of study drug. * Patients aged 18 years or above at screening. Exclusion Criteria: * Crohn's Disease Related * Fistula abscess present at screening. * Stricturing type disease with symptoms or signs of non-inflammatory mechanical obstruction or bowel perforation in last 3 months. * Short bowel syndrome. * Functional colostomy or ileostomy (note: patients who have had a temporary stoma in the past, which has been reversed, are eligible to enter the study). * Positive stool laboratory results for enteric pathogens.