Brazilian swim star Cesar Cielo fought to avoid a doping ban that could wreck his Olympic chances in a case that has rocked the sport and overshadowed the world championships. The 24-year-old Olympic and double world champion went into a hearing in Shanghai on Wednesday, along with three team-mates who also tested positive for a banned diuretic in May but were let off with just a warning. Cielo, wearing a dark suit and sunglasses, smiled but did not speak to media as he arrived for the Court of Arbitration for Sport (CAS) tribunal, where he is represented by celebrated doping-in-sport lawyer Howard Jacobs. The Brazilian has blamed a contaminated food supplement for testing positive for furosemide, which is sometimes used to mask other drugs and is proscribed by the World Anti-Doping Agency. Governing body FINA has tightened its anti-doping procedures at the Shanghai championships by reintroducing blood testing, alongside urine sampling, for the first time since 2005. FINA executive director Cornel Marculescu also issued a sharp reminder that athletes had to take responsibility for all the vitamins and food supplements they consume. \"Always the issue is that when you take a supplement, you must be sure that you have done everything (to check it is safe),\" Marculescu said last week. Governing body FINA has asked CAS to suspend the swimmers and annul their results since May, which would endanger their chances of taking part in next year\'s London Olympics. The court has promised to hand down a judgement by Friday at the latest. The world championships swimming events start on Sunday. Athletes at the world championships have been urged only to eat at designated restaurants in Shanghai to avoid meat contaminated with banned clenbuterol, which Chinese farmers sometimes feed to pigs and cattle.