The Lotto-Soudal rider crashed around 60 miles from the end of Monday’s stage from Chorzow to Zabrze in Poland.
He was taken by helicopter to hospital where he later died from his injuries.
His team later confirmed on Twitter: “The biggest tragedy possible that could happen to the family, friends and teammates of Bjorg has happened... Rest in peace Bjorg.”
The Tour of Poland added: “There are no words to describe this tragedy. Bjorg Lambrecht passed away during following an accident on today's stage.
“Our thoughts are with his family, friends, team and the whole cycling community.”
Race director Czeslaw Lang later elaborated on Lambrecht’s fatal accident. “It wasn't a high-speed descent. It was a straight and wide road,” he said.
“He had a moment of hesitation, he left the asphalt and the tragedy occurred.
“The injuries were so serious that we called for a helicopter and an ambulance. He was transported to hospital. His heart started beating again but he sadly died during the operation.”
Lambrecht was competing in his second season on the WorldTour and had been regarded as one of his nation's most promising young talents.
He won the Under-23 version of the prestigious Liege-Bastogne-Liege race in 2017 and finished second in the U23 World Championships last year.