The singer is said to have commissioned a "large, sparkly and expensive" engagement ring from a leading London jewellers so he can pop the question to The Saturdays member.
The pair have been dating since February last year but briefly split in November.
They reunited two months later and have since exchanged promise rings as a symbol of their commitment to one another.
A source told the Daily Star Sunday: "Marvin has already had the ring made and is now just waiting for the right time to propose.
"He was thinking about going for a Christmas Day proposal but he's not sure he can wait that long."