Running at the low-key Papaflessia meeting in Kalamata, Greece, the disgraced sprinter clocked 10.25sec to steal victory ahead of American Mike Skiles, who was given the same time.
The time was outside the Olympic Games qualifying standard and puts Chambers fourth in this season’s British rankings. It was, however, enough to qualify him for the Olympic trials, which begin in Birmingham on July 11.
That will probably be enough for Chambers, 30, to launch a legal challenge against the British Olympic Association’s by-law banning serious drug offenders like him from representing this country at the Games.
A bullish Chambers said: "I’m just pleased to be here competing, first and foremost. It’s great to get back into competition in the 100 metres. I’m sticking around. Beijing is still there for me. I’ll be there, don’t worry."
Nick Collins, the Leeds-based lawyer who has represented Chambers since the sprinter returned to the sport for a second time this year following his two-year doping ban, was as chipper as his client.
Collins said: "Let’s hope Dwain is right. It was a good time though there was no sort of target for him.
"We’ve already issued proceedings and have been in touch with the BOA’s solicitors telling them we intend appealing on Dwain’s behalf. Hopefully, the case may even be heard in the High Court before the end of next week. "
A spokesman for the BOA said: "Our position hasn’t changed. We’ll defend the by-law vigorously."
Chambers was among the first high-profile athletes to test positive for the designer steroid THG, the drug at the centre of the BALCO doping scandal in the United States.
He claims to have received a lot of support from the public for a stance that many believe will seriously undermine this country’s fight against doping.
"What has happened has happened and I can’t change it now," Chambers said. "For me it’s important that the public support me and I’ve been very encouraged by that."
He added: "I’m optimistic I will be in China. The BOA ruling is unfair and this only exists in Britain.
"I don’t want to put my country in a difficult position. I’m doing this because I believe I can do well in the Olympics. If I didn’t believe that, then I wouldn’t waste my time and the time of my lawyers.
"I believe that I can be in the first three in Beijing. I can win a medal."