Neil Jackson, who plays Harry Spargo in the series, confirmed the news on his Twitter page.
"Such a shame. I have just heard that the BBC will not be making any more Upstairs Downstairs. I loved the show and will miss... [it] greatly," he wrote.
Upstairs Downstairs ended its first series with almost 7m viewers in 2011. It returned with 6.53m viewers in February of this year.
Jackson recently won a lead role in NBC pilot Notorious, created by Friday Night Lights writer Liz Heldens.
He has previously told Digital Spy that season two of Upstairs Downstairs was "infinitely bigger", describing it as a "fantastic show".