University College researchers believe the painting, found in Streatham, dates back to the queen's 16th-Century reign.
"An inscription on the portrait appears to have been put on at the same time as the paint, which was obsolete by 1700," said researcher Libby Sheldon.
Lady Jane was beheaded at the age of 17 following her short reign in 1553.
'Composition match'
The Protestant Lady Jane was the grand-daughter of Henry VIII's Catholic sister Mary, who proclaimed herself rightful queen under the terms of Henry's will.
It had long been thought that she was the only English monarch since 1500 of whom no contemporaneous portrait survived.
Painting experts studied the portrait for five years before concluding that it "appeared to be" painted during Lady Jane's lifetime.
"We can never be 100% certain," said Ms Sheldon, "but the composition of the lead tin yellow paint and its chalk background matches other paintings from that period."
Owned by a private collector, the painting had been restored by collector Christopher Foley, who believed it was a 16th-Century copy of a lost original.
Further examination found that the portrait had originally been scratched "vigorously" around the subject's eyes and mouth.
"This may have been done for a political or religious purpose, as it would have forced its owners to remove the painting from public display and restore it," said Ms Sheldon.
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