The 'Never Let Me Go' actress – who was first spotted with the folk musician at an intimate concert in Nashville in February – was showing off her engagement ring to guests at Babington House in Somerset, South West England, over the weekend.
A witness told The Sun newspaper: “Carey looked like the cat who had got the cream. She was wearing a beautiful ring and didn't seem to mind who saw it.
"They were stuck to each other like glue. She was doing most of the talking and he was hanging on her every word."
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Carey, 26, and Marcus, 24, have been inseparable since they began their relationship, often spending time together at the 'Roll Away Your Stone' singer's London home, as well as enjoying a break together to the Orkney Islands in Scotland in March and most recently Carey joined Marcus on the Mumford & Sons' 'Railroad Revival' tour in Arizona.
Carey, 26, has previously dated ‘Transformers’ star Shia LeBoeuf, while Marcus, 24, split with fellow folk singer Laura Marling not long before meeting Carey.