Powell tested positive for a stimulant at last month's national championships, his agent told Reuters on Sunday.
Powell, who has never won an individual global sprint title, held the 100m world record between 2005 and 2008 when his then-best of 9.74 seconds was broken by current record holder and fellow-Jamaican Usain Bolt.
In a statement, Powell said: "I will confirm that a sample I gave at the national trials in June earlier this year has returned 'adverse findings'. The substance oxilofrine (methylsynephrine) was found, which is considered by the authorities to be a banned stimulant.
"I want to be clear in saying to my family, friends and most of all my fans worldwide that I have never knowingly or wilfully taken any supplements or substances that break any rules.
"I am not now - nor have I ever been - a cheat," he added.
"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."
Olympic 4x100m relay silver medallist Sherone Simpson said on Sunday she had tested positive for a stimulant at last month's national championships.
Simpson, 28, was a member of the Jamaican team that finished second in the 4x100 relay at last year's London Olympics.
She finished equal second in the 100 metres at the 2008 Beijing Games and won a gold medal in the 4x100 relay four years earlier in Athens.
"This is a very difficult time for me," Simpson said in a statement to Reuters. "I was notified on July 14, 2013 that my urine sample taken at the National Senior Championship, June 21, 2013 after the 100m finals returned a positive analytical finding for a stimulant, oxilofrine (methylsynephrine).
"As an athlete, I know I am responsible for whatever that goes into my body. I would not intentionally take an illegal substance of any form into my system."
The sources said two of the athletes were sprinters while three competed in field events. They said one of the athletes was a junior.
Jamaican Anti-Doping Commission chairman Herb Elliott confirmed the body had received "reports of adverse analytical findings from "A" samples".
"The process of the result management has commenced in accordance with the JADCO Anti-doping rules," he told Reuters.
"We cannot disclose any further information until the athletes have responded to notification of the "A" sample."
The managers for Bolt and world 100m champion Yohan Blake said their athletes were not involved. Blake did not compete at the championships because of injury while Bolt won the 100m.
Last month Jamaica's most successful female athlete Veronica Campbell-Brown tested positive for a diuretic, which can be used to mask the use of performance-enhancing drugs.
The twice Olympic 200m champion has been suspended by the Jamaican Athletics Administrative Association pending the outcome of a disciplinary panel hearing.
- Sports & Recreation
- Athletics, Track & Field
- Usain Bolt
- 100m world record
- Sherone Simpson