Dame Joanna Lumley has warned of a "crisis hidden in plain sight", with 1.5 million older people set to spend Christmas alone.
Age UK spoke to more than 2,600 people and found 11% will eat dinner alone on 25 December, while 5% will not see or speak to anyone the whole day.
Applied to the overall population, the findings suggest 1.5 million people will eat alone at Christmas, according to the charity.
Dame Joanna said the "silence can be deafening" for those left isolated and called it "a crisis hidden in plain sight".
The actor and campaigner is now joining other luminaries including Dame Judi Dench, Brian Cox and Miriam Margolyes to back Age UK's campaign against loneliness.
The charity says its volunteers made more than 70,000 minutes' worth of calls to people during Christmas week last year and is urging people to donate.
'A tragedy we don't talk about enough'
Age UK said it also supports coffee mornings and festive lunches to give lonely people the chance to enjoy in-person interaction.
Dame Judi said: "For so many older people, Christmas can be a time of silence - days without conversation or company."
Succession star Brian Cox called the issue "a tragedy we don't talk about enough".
He said: "Far too many older people are left spending the season in silence, when it should be a time of warmth, connection and joy."
Margolyes, of Harry Potter fame, added: "Growing older shouldn't mean disappearing into the background, we need to be seen, heard and celebrated.
"That's what Age UK is striving for - they're changing how we perceive age."
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The charity's chief executive, Paul Farmer, said: "Your donation could bring comfort, friendship, and care to an older person facing loneliness this winter.
"From friendly, weekly calls to local lunch clubs, we're here to make sure no one spends winter alone. But we can't do it without you."
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