There's nothing holdin' them back! Shawn Mendes and Camila Cabello's relationship is heating up and "moving quickly."

After years of friendship and musical collaborations, the "Señorita" singers have officially sparked a romance. After putting on a steamy display in their music video in June, Mendes, 20, and Cabello, 22, sent relationship speculation into overdrive when photos emerged of them holding hands over the Fourth of July holiday.

Days later, the duo continued to fuel romance rumors with a PDA-packed brunch in West Hollywood, Calif. Over the weekend, Mendes and Cabello were also spotted enjoying an intimate breakfast together in San Francisco.

So what's really going on between the superstar singers?

"It already feels to them like they've been together for a lot longer than they have," a source tells E! News. "They've known each other and been a part of each other's lives for a while so it's all very natural and moving quickly."

Shawn Mendes, Camila Cabello

CPR/D.Sanchez / BACKGRID

"The timing wasn't right before but it is right right now," the insider continues. "They are going for it without holding back. This step in their relationship is exciting for them both. They haven't wanted to be apart at all. It's 24/7 and all out at this point."

The source adds that even though Mendes is currently on his tour, Cabello "wants to be there and not leave his side."

"Neither of them want to be away from each other," the insider tells E! News. "She has the opportunity to be there with him and she's taking it."

Prior to her romance with Mendes, Cabello was in a relationship with Matthew Hussey. The duo's split was announced in June. Mendes was previously linked to Hailey Bieber.

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