Although the couple recently split for six weeks, their time apart only made them realise how much they love each other - prompting them to talk about marriage, with Dougie, 22, planning to propose any day.
A source told the Daily Star newspaper: "They split up because their hectic schedules meant they were finding it hard to find time to see each other.
"But while they were apart they spent most of the time on the phone to each other. They realised they can't live apart any longer.
"They plan to get engaged and hope to be married before Christmas. They are determined not to let work get in the way."
The pair originally separated in March after a year of dating and Frankie, 21, moved out of the apartment they were sharing.
Following a tense meeting at London's exclusive Mayfair hotel, the couple publicly announced they were back together.
Frankie said: "Everyone keeps saying how pleased they are we're back together."
McFly are currently finishing their fifth album, while The Saturdays are also in the studio, recording their third LP.