A quarter of homeless people face this Christmas alone, a charity has warned.
Thousands of people will spend the festive period on the streets.
As families across Britain gather to celebrate, more than two-thirds of homeless people surveyed admitted they would spend Christmas with neither relatives nor friends, Crisis found.
Four thousand homeless people are expected to visit the Crisis at Christmas centres which open today and run until the end of the month, providing shelter, food and company for those who find themselves without a home.
With a focus at Christmas on being with loved ones, this time of year can be especially isolating for the homeless, the charity said.
Almost three-quarters of those questioned said they felt ashamed of being homeless, while 44% said they felt undeserving of help.
More than a quarter of people said they had formed an unwanted sexual relationship just to keep a roof over their head, Crisis found from surveys with 506 people at day centres across England, Scotland and Wales.
Jon Sparkes, chief executive of Crisis, said: "Christmas should be a time for family and friends, for warmth and celebration, yet for homeless people it can be one of the hardest periods of the year - a cold, lonely experience to be endured rather than enjoyed.
"That's what makes our work at Christmas so important."
But while loneliness can become more acute at Christmas, the tragedy of being homeless is one that can affect people at any time, he added.
"Homelessness is a desperate, isolating experience that destroys people's confidence and self-esteem and makes it even harder for them to get help," he said.
"We already know that homeless people are over nine times more likely to commit suicide, and there can be little doubt that loneliness plays a major part in that tragedy."
Carlos Blanco found himself sleeping near the bins in the car park of his flat when he was evicted just two weeks before Christmas last year.
The 44-year-old had been working as a swimming teacher but did not have regular hours and ended up struggling to pay his rent.
He was directed to Crisis, a charity he had never heard of, to spend the Christmas period at one of their centres.
"For me, being in the situation I was in, they gave me the holiday of a lifetime," he said.
"It was all about being with people.
"I got the chance to meet people and make friends and the volunteers are amazing."
Having spent around six months of the past year on the streets, Mr Blanco said he was "relieved" to have recently been allocated a bedsit in London, and was now on the hunt for a job.
This year he will be a volunteer, rather than a guest at one of the Crisis centres in the capital.
He said: "I said if I was able to I would come back and give my time.
"None of this would have happened if it wasn't for Crisis and the help they gave me."
The father-of-one said the work the charity did all year round was invaluable.
"Any day is an awful day to be homeless, not just Christmas," he said.
"The services Crisis offers to people - the courses, advice and support - are really important and they go on every day."