The chart-topping singer paid tribute to the late George Michael at a Crisis shelter in west London, where he performed the festive-themed tune as well as some of his own biggest hits.
One volunteer at the Shelter posted a picture of himself alongside the Coldplay frontman on Instagram and captioned the image: ''That moment when Coldplay's Chris Martin drops into the Crisis at Christmas shelter, with no fuss, no fanfare and no press, to help make tea and coffee and also play guitar so the guests can have a sing song.
''Thank you for coming down and getting stuck in.''
The appearance at the homeless shelter came shortly after it was revealed that Chris gives 10 per cent of his million pound earnings to charity.
The 39-year-old star's mother Alison kept back 1p of his 10p pocket money while he was growing up to teach him the importance of sharing, and it appears the life lesson stuck with him as Chris now makes sure he gives away some of his income to help others.
Appearing on an Australian TV show, bandmate Jonny Buckland said: ''Ten per cent of Chris' pocket money goes to charity.''
Chris - who is worth an estimated £70 million - added: ''I used to get 10 pence pocket money. The first time she gave me nine, I was like, 'What is this?' [And my mum was like], 'Get used to it, son.' Mum still gives me three pounds a week.''
Chris' bandmates also donate money to charity, giving away ten per cent of their own cash to 28 different charities every month.
Chris explained: ''It's very important for us [to give money to charity regularly]. It drives us on. We don't talk about it that much but you asked. It is true!''
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