The 33-year-old singer and Melanie, 47, have made a conscious effort to keep their romance quiet, but the duo - who have bonded over their mutual passion for keeping fit - have been on a series of hush-hush dates in 2017.
A source explained: ''Mel loves a younger man and really fell for Olly when he got into shape late last year.
''They were initially introduced through a mutual friend. They started texting for ages and some of the messages ended up getting quite saucy.
''They were bonding over her fitness programme. He was a fan of her gym selfies and thought that she had an amazing body.
''Olly was getting in shape at the time and Mel really loved his new fit bod.
''They ended up meeting for a date and have had a whole series of secret dates at his house where she will sometimes stay overnight.''
Although their romance had already endured some ups and downs, it's claimed that the high-profile pair share a ''real physical attraction''.
The insider told The Sun newspaper: ''They have both been determined for nobody to find out. It's been a bit off and on and there's a real physical attraction between them.''
Olly and Melanie have previously traded compliments over social media.
On one occasion, Melanie posted a picture of Olly's track 'Flaws' and captioned it: ''Love this track @ollymurs you sound like an angel makes me want to cry x (sic)''
The comment prompted Olly - who has not been in a serious relationship since splitting from his long-term girlfriend Francesca Thomas in 2015 - to respond by saying: ''Awwww thanks babe (sic)''
'Smalls Change (Meditations Upon Ageing)' arrives in April.
The two awards have made for a great 72nd birthday present for the country music icon.