The News of the World reports that the pair were caught by members of the security team after a kissing game went too far.
A source said: "A group of contestants were mucking about but Katie and Matt took things further. It was obvious something steamy was going on.
"Matt is known as the ladies' man in the house and he's been getting closer to Katie over the last few days. He loves her confidence and thinks she's gorgeous. Katie is the sort of girl who needs the attention of a bloke to feel contented.
"They'd been mucking around and ended up kissing during a silly game a few of them were playing. But there was an obvious attraction and they ended up in the same bed."