Demi Lovato's engagement ring reportedly cost more than $3 million.

The singer and former Disney star, 27, became engaged last week to her actor partner Max Ehrich after a whirlwind five-month romance.

According to TMZ, the stonker of an engagement ring her beau popped the question with was designed by jeweller to the stars Peter Marco, and could be worth up to $3.5m.

With an emerald-cut 10-carat diamond set with two trapezoid-shaped diamonds on either side, and a platinum or white gold band, the publication reports the total carat weight could be between 9.5 to 11.5, or even up to 20.

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The publication claims Ehrich, 29, ensured the design was one of a kind by opting for a distinctive elongated stone following a series of discussions with Marco.

Separate sources have come up with a seven-figure estimate for the ring, with one claiming it bears a distinct resemblance to the creations of celebrated jeweller Harry Winston.

"A ring of this size would retail close to $1.3 million," WP Diamonds Brown CEO Andrew Brown told E! News.

"If it's indeed a Harry Winston, the ring would retail closer to $3.4-4 million."

Meanwhile, Brilliant Earth's SVP of merchandising and retail expansion Kathryn Money told the outlet that "depending on the quality and specific characteristics of the centre gemstone "the ring's cost could be estimated between $627,000 and $1.2 million".

Lovato announced the exciting engagement news last week in a series of sweet snaps of the couple kissing on a beach in Malibu at sunset.

She wrote: "When I was a little girl, my birth dad always called me his 'little partner' – something that might've sounded strange without his southern cowboy like accent.

"To me it made perfect sense. And today that word makes perfect sense again but today I'm officially going to be someone else's partner."

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Lovato went on: "I knew I loved you the moment I met you ... You never pressure me to be anything other than myself. And you make me want to be the best version of myself.

"I'm honoured to accept your hand in marriage. I love you more than a caption could express but I'm ecstatic to start a family and life with you.

"I love you forever my baby. My partner. Here's to our future!!!!"

Lovato, who has been open about her struggles with drug addiction, eating disorders and mental health in the past, then thanked her fans in the comments section, posting: "To all of my friends and fans commenting – I F***ING LOVE YOU – THANK YOU FOR SUPPORTING MY HAPPINESS."

Sources claimed the couple began dating in April, three months after she ended her relationship with model Austin Wilson, who she was first linked to in November, and have been isolating at Lovato's LA mansion.