Tanya Roberts Cause of Death Was Likely UTI, Not COVID

Tanya Roberts' feared she had contracted COVID-19 before collapsing at home ... but doctors believe a serious infection -- not COVID -- is what killed the actress.

The late actress's partner, Lance O'Brien, tells TMZ ... Tanya said she felt short of breath, tired and had tightness in her stomach just days before her collapse on Christmas Eve. Lance says when Tanya collapsed he helped her up ... and realized she had virtually no strength. An ambulance arrived and took her to the hospital, giving her oxygen along the way and helping her to regain consciousness.

Once Tanya was hospitalized, Lance says her condition quickly deteriorated -- which Lance himself recounted on camera Tuesday. BTW, we're told she was tested for COVID several times while in the hospital, but all of the tests came back negative.

So the question remains ... what could have taken her life so quickly? Lance tells us that doctors informed him it might be related to a UTI that spread to her kidneys and liver. We're told Tanya also had Hepatitis C, and doctors told Lance that was causing complications as well.

An autopsy has yet to be performed. Tanya was 65.