Hundreds of volunteers with homelessness charity Crisis have been working to turn five London premises - including one in SE1's Mandela Way, off the Old Kent Road - into Christmas shelters.
They expect to provide food and shelter for up to 1,000 people at the five Crisis shelters, which will be open 24 hours a day from 23 to 30 December.
The charity said increasingly it was providing for what it calls the hidden homeless, rather than rough sleepers.
Every year, 2,500 volunteers give up their Christmas to run the shelters, working on an eight-hour shift system.
The Open Christmas provides more than basic food and shelter – there is also advice on housing benefit, medical services, counselling and help with drug, alcohol and mental health problems.
Crisis chief executive Shaks Ghosh said: "Increasingly now we're working with people who are still homeless but living in emergency shelters, grotty B&Bs.
"This gives them somewhere to come, obviously to get their services, but also that important companionship."
The charity estimates there are about 400,000 single homeless people in England, living in shelters, bed-and-breakfast and even sleeping on friends' floors.
Last year Crisis operated an internet cafe for the first time, with computer experts, who were once rough sleepers themselves, assisting users to send emails to friends and family around the world. The service was fully booked for all the 306 hours the shelters were open.
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