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The 36-year-old 'Wildfire' hitmaker is expected to propose to Roar singer, who split from British comedian Russell Brand in December 2011 after 14 months of marriage, in the near future.
A source told E! News: "It's inevitable."
The insider said that the couple are "very serious" and added: "Everyone wants them to get engaged. They are so perfect together."
Katy recently gushed about her feelings for the singer-songwriter, whom she first started dating in June 2012.
She said: "I actually am dating my crush. I had a crush on him for a long time."
She also insisted she loved being married even though she got a divorce.
She said: "Of course. I really loved it. It was a new, fun, exciting journey. But I like how I am now, and I think all things happen for a reason."
John, who previously dated Jessica Simpson and Jennifer Aniston, presented the 29-year-old star with a heart-shaped ruby ring for Valentine's Day in Santa Barbara, California, earlier this year but a source insisted at the time that they weren't ready to tie the knot.
The insider said: "It's a one-of-a-kind Daniel Gibbings ring featuring a 12 carat ruby high-dome cabochon heart shaped centre, accented by two teardrop faceted sapphires and rubies, set in 20-carat gold."
