The soap star passed away at home last week in Jupiter, FL after succumbing to her longtime battle with progressive supranuclear palsy, a rare neurological disease ... so says the East Hampton Star, which first reported her death.

The outlet also reports her brain tissue is being donated to the Mayo Clinic for research. On her health struggles, her husband of 47 years, James D’Auria added ... "Her courage and bravery tried in vain to fight the disease."

Jennifer was best known for playing Gwen Sherman on 'Y&TR' for a couple episodes -- and notably starred opposite Lucille Ball in 1968's "Yours, Mine and Ours," playing the comedic legend's teen daughter.

The Wales-born actress actually almost had her big break in 1967's "The Graduate" after impressing director Mike Nichols ... but ultimately lost out on appearing in the critically-acclaim flick due to immigration issues.

Jennifer still found success in Hollywood, however ... her notable credits include "Hawaii Five-0," "Nero Wolfe," "The Mary Tyler Moore Show," and "Another World."

Her last listed credit was in 1992 for the ABC soap opera "Loving," in which she played Dr. Hennessy for 2 episodes.

Jennifer is survived by her husband, as well as her brother, Kenneth Leak. Her ashes will be buried in a cemetery located in her home country of Wales. She was 76 years old.