Kevin Clash -- the man known as the voice of Elmo -- has taken a leave of absence from Sesame Street in the wake of allegations he had a sexual relationship with a 16-year-old boy, TMZ has learned ... allegations Clash adamantly denies.

We've learned ... Sesame Workshop lawyers recently met with a 23-year-old man who claims he and Clash began a sexual relationship 7 years ago ... when he was 16 and Clash was 45.

Clash has acknowledged to TMZ he had a relationship with the young man -- but insists it only took place AFTER the accuser was an adult.

Sesame Workshop honchos tell TMZ ... "In June of this year, Sesame Workshop received a communication from a young man who alleged that he had a relationship with Kevin Clash beginning when he was 16-years-old. This was a personal relationship, unrelated to the workplace. We took the allegation very seriously and took immediate action."

Read more: [link to www.tmz.com]