Britney Spears' Ex Jason Alexander Allegedly Threatened to Break Woman's Neck

Britney Spears' ex, Jason Alexander's stalking victim claims she has a very good reason to fear him -- she told cops Alexander threatened to snap her neck.

According to new legal docs obtained by TMZ, the woman, who's identified as Lisa, claims Jason issued the threat last January ... saying he would break both her neck and that of her male friend.

In the docs, she says Jason approached that same guy friend in July and again threatened to snap his neck if he didn't stop texting Lisa.

It didn't end there ... Lisa filed another report in October claiming Jason came on her Tennessee property after being told he was not welcome. He allegedly admitted to following her home, claiming he was just making sure she got home safely.

According to the docs, Jason returned to Lisa's property in November and argued with her. This time she called cops, but he bounced before they arrived.

TMZ broke the story ... Jason was arrested Dec. 30 and charged with violation of a protective order and aggravated stalking. He pled guilty to misdemeanor stalking last week and is now on probation.

The protective order was issued Dec. 10 and it barred Jason from contacting Lisa -- but the new docs say he emailed her on Christmas, and also brought presents for her and her kid to the Franklin Police Department ... so cops could give them to her on his behalf.

That creepy Christmas gesture violated the protective order, and that's why he got dinged.

Jason's most famous for dating Britney back in 2004, and abruptly getting married in Las Vegas. That union lasted all of 55 hours before it was annulled.