Even when judged against the inglorious and grubby scandals of the past 25 years this will go down as a dark day for athletics. To have Tyson Gay, the fastest man in the world this year, test positive for a banned substance is catastrophe enough. But the revelation, a few hours later, that Asafa Powell, the third quickest 100m runner in 2013, also failed a test will send even the sport's most ardent defenders into shock.

Both men have insisted that they are not cheats. But under athletics' strict liability laws that is no defence, as both Gay and Powell recognise. There is no knowing yet what the helter skelter of the next few days will throw up. But in the public's mind they join a long list of high-profile sprinters who have failed drugs tests, from Ben Johnson to Marion Jones to Justin Gatlin.

Powell was one of five Jamaicans to test positive at their national championships last month, one of whom was the Olympic 4x100m relay silver medallist Sherone Simpson. Both their samples were found to contain the banned amphetamine oxilofrine. Gay, meanwhile, tested positive for an unnamed substance on 16 May.

It was also revealed on Sunday night that a member of Powell's support staff had been arrested in Italy and that materials relating to the investigation had been seized. Powell's agent, Paul Doyle, would not be drawn on the reports but said they were "working closely with the World Anti-Doping Agency to make sure everything is considered and everything is transparent."

The news means that both Gay and Powell are out of the world championships in Moscow next month. Now, inevitably, the 100m loses its lustre – while the sport loses another small chunk of the trust it has built up since the last scandal.

Gay, the second fastest man of all time having set the American record of 9.69 seconds in 2009, said he took full responsibility for his positive test. But he claimed it was a "type of accident" – before hinting that a friend had let him down. "Sometimes a human being naturally, generally trusts somebody," he said. "That's what people do."

The American had struggled since 2009 with groin and hip injuries, the latter of which required surgery, but after an injury-free winter he had run 9.75sec this season and also won the 100m and 200m trials at the US championships. Everything looked set for an intriguing showdown with Usain Bolt at the world championships in Moscow. Now he must take his punishment.

"I will take whatever punishment I get like a man," Gay said.

"I do realise and respect what I put in my body and it is my responsibility. I'm going to be honest with Usada, about everything, everybody I've been with, every supplement I've ever taken, every company I've ever dealt with, everything."

Gay was once part of Usada's programme called My Victory, in which athletes pledged to stay clean. In it, he said: "I compete clean because I really believe in fairness and, besides that, my mom would kill me! Just being honest."

In a statement Usada claimed that it appreciated Gay's decision to "voluntarily remove himself from competition while the full facts surrounding his test are evaluated".

Meanwhile the news that Powell has tested positive for a banned substance surprised many in athletics, especially given the Jamaican's vocal anti-drugs stance over many years. Powell maintained that he had "never knowingly or wilfully taken any supplements or substances that break any rules" and said that his team would launch an investigation. "I am not now – nor have I ever been – a cheat," he said. "My team has launched an internal investigation and we are co-operating with the relevant agencies and law enforcement authorities to discover how the substance got in my system.

"I assure you we will find out how this substance passed our rigorous internal checks and balances and design systems to make sure it never happens again," he added. "My attitude towards doping regulations and testing is well-known and I willingly give samples whenever requested. This result has left me completely devastated in many respects. Professionally, this finding fully negates any possibility of me being a part of Jamaica's contingent of athletes competing at the world championships in Moscow later this summer."

The news of the positive tests was greeted with dismay at the British Championships in Birmingham. Colin Jackson, the former 110m hurdles world record holder, admitted it would have "a huge effect on the sport".

"You can imagine it takes away faith from the general public which we were building nicely," he said. "The good side is at least our sport is still testing hard. It doesn't matter what level you're at – if you're taking drugs you will get caught. But for the sport it's absolutely awful."

It was not a struggle to find the positives for athletics on Sunday night – they were, it seemed, everywhere. But one beneficiary of the spate of drugs tests is likely to be the British 100m sprinter James Dasaolu, who ran a startling 9.91sec in the 100m at the UK trials on Saturday. That, remarkably, is now the second fastest 100m time of any athlete competing at the world championships – and means he is now a genuine medal contender in Moscow.

Certainly Neil Black, the performance director of UK athletics, made no bones about how these drug tests had affected Dasaolu's chances. "It changes everything," he said. "We see him as a genuine finalist and there are one or two people who are not there, or who are not at their best, so we are pretty excited."

Black also stressed that he would redouble his efforts to ensure that no British athletes were caught taking supplements that might contain banned substances. "I'm not sure that there is anything else that we can do," said Black. "But we will see if we can make things even clearer in case there are any crazy things out there."