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Former World Anti-Doping Agency president Dick Pound believes it will be "very hard" to trust any Russian athlete at the Olympics this summer if the country succeeds in overturning its ban from international competition.
Pound's comments were supported by world marathon record-holder Paula Radcliffe, who said she remained "very suspicious" of Russian claims it had solved its doping problems.
"We're all suspicious that they can do what they need to do to assure us the entire team is clean to compete fairly in Rio," said Radcliffe.
Pound chaired the independent commission that investigated claims of systemic doping in Russian athletics, a process that started when whistle-blowers told German broadcaster ARD about the scale of Russia's cheating in late 2014 and finished with the Olympic powerhouse being thrown out of the sport last November.
Since then, a WADA "task force" has been sent in to overhaul Russia's anti-doping system in an attempt to bring the country back into the fold in time for the Olympics.
A decision on Russia's return will be made by the sport's governing body, the International Association of Athletics Federations (IAAF), at a meeting of its 27-member council in Vienna on June 17.
But Pound, who earlier this year suggested Russia was just "rearranging deck chairs on the Titanic", remains sceptical that the country has acted quickly enough to fix its problems.
Speaking at the Sport Resolutions conference in London, the 74-year-old Canadian said: "When we delivered our report back in November, we said they could get back in if they went full speed ahead to do the job properly but it's up to them.
"There will be a lot pressure to get them back in and, from a system point of view, it would be nice to have everybody at the Games.
"But whether t hat makes sense in terms of the changes they've made remains to be seen.
"I think there's still some elements of denial (in Russia)."
Pound added that if he were in charge of the International Olympic Committee, he would find it difficult to say with complete conviction that any Russian athletes in Rio would be clean.
He said: "If I were IOC president and we let the Russians back in, are we absolutely certain that every Russian athlete isn't doping and everything has changed.
"I think it's very hard to say that."
The former Olympic swimmer's comments about the growing feeling that Russia is winning the argument over reinstatement were backed up by UK Athletics boss Ed Warner.
He voiced his unease at the thought of seeing Russian athletes lining up in events they have dominated in recent years, such as the women's middle-distance races, knowing they may still be benefiting from doping products they have only recently stopped using.
Radcliffe has raised this issue many times in the past and the 42-year-old Briton repeated her calls for longer drugs bans.
But she also wants to widen the investigation into Russian athletics to include other sports and countries.
"It's not fair that a Russian swimmer who probably had access to the same drugs is there and an athlete isn't," she said.