(BBC) Athletics has "come through the worst" of its doping problems and the London 2017 World Championships will be the "cleanest ever", according to the event's director Niels de Vos. The sport remains dogged by allegations of doping, with Russia and Kenya facing bans from this summer's Olympics for failing to tackle cheating athletes. But UK Athletics chief De Vos said: "I think we're through the worst of it. "I think without doubt 2017 will be the cleanest championships ever." De Vos, 49, added: "I would be very hopeful 2017 will be a very clean games but it's impossible to say with certainty and that's a shame." Russia was suspended from world athletics in November and must now convince the sport's governing bodies it has reformed its doping polices to be reinstated at the Olympics. Three IAAF officials were banned for life in January for breaching anti-doping rules while Kenya has missed two deadlines to prove it can combat drug-taking.