TMZ has learned … LeBron James and his mother Gloria James are being sued for millions by a man who claims he tried to prove he’s the NBA star’s biological father — but LeBron and his mom tampered with the evidence in a diabolical cover-up.

The lawsuit is explosive — in it, 55-year-old Leicester Bryce Stovell claims he met Gloria in a D.C. area bar in 1984 … and had unprotected sex with her on the night they met. He later found out she was 15-years-old at the time … he was 29.

According to the docs — filed recently in federal court — Gloria has spent the rest of her life shielding LeBron from the truth.

But the man making the claim isn’t some schmuck — dude is a Princeton graduate … who earned a law degree from the University of Chicago … and then became a Senior Legal Advisor for the U.S. Securities and Exchange Commission.

In his suit, Stovell claims he has a very clear recollection of the night he had “consensual sexual relations” with Gloria — in fact, he even remembers apologizing for his sub-par performance.

Leicester claims Gloria went back to her hometown in Ohio after they hooked up — but returned several months later to inform him she was pregnant with a boy named LeBron … but she never specifically identified the father.

Leicester — a self-proclaimed high school star athlete — says he remembers telling Gloria, “Well, if he’s mine, make sure he plays basketball.” He claims that was the last time he saw Gloria for more than 20 years.

According to the docs, Leicester claims he reconnected with Gloria over the phone in 2007 — when he began to think he could be LeBron’s father … partly because they look almost identical. Leicester also notes that when he saw LeBron at an NBA game he was, “struck by our similarity in appearance … James and I are nearly if not identical height and pigmentation and have very similar athletic body types.”

Leicester claims during the 2007 phone call — Gloria denied ever meeting him — threatened to have him physically harmed — and told him, “LeBron’s money is for his children.”

A few days after the phone call, Leicester claims LeBron — through his lawyer — agreed to take a DNA paternity test.

Leicester claims he was given results that showed he was not LeBron’s dad … but several months later, he began to suspect that the results were tampered with … and Gloria and LeBron were behind the fraud.

So why would LeBron participate in such a cover-up? Leicester claims it’s all, “due to anger at perceived abandonment and conflict arising from his image as a successful fatherless child from the projects.”

He also claims that Gloria had “fabricated” a story about LeBron’s real father for the “one dimensional commercial image-making purpose … to make [LeBron’s] life suggestive of Spike Lee‘s popular 1998 sports drama film, ‘He Got Game.’“

Leicester is now seeking $4 million from both LeBron and his mother — claiming fraud, defamation and misrepresentation.

And check this out — Leicester also points out that LeBron named his 2nd son “Bryce” — and claims it’s no coincidence. As we said, Leicester’s middle name is Bryce.