The couple were photographed together at the Book of Mormon show in London last week, and there is growing speculation that their relationship many have been going on for months.
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Pictures posted on Twitter show One Direction star Harry sat next to Cara, 21, in the dress circle before the performance starts.
Harry, 19, can be seen on his phone, while Cara – her lion finger tattoo clearly visible – concentrates on her drink.
According to Look magazine, once the lights dimmed, the couple cosied up together and were whispering in each other’s ears.
It's not the first time they have been romantically linked. They were rumoured to be dating back in 2012, shortly before Harry went public with Taylor Swift.
"They have great chemistry," a source told the magazine. "Sure, they've always agreed to keep things super-casual and private because of their crazy worlds, but Harry genuinely likes Cara a lot – he has done since they met three years ago.
"They've actually had an on/off thing going all this time when they're in the same city, but they’ve never had the chance to give a real relationship a shot".