Scott Disick and Sofia Richie. Picture: Instagram

Sofia Richie left a sweet message for Scott Disick as she marked his 35th birthday.

The 19-year-old model took to social media to mark her boyfriend's big day by sharing a picture of them enjoying the sun and sea on vacation.

She wrote on Instagram: "Happy Birthday babe! Thank you for being you. Love you."

Happy Birthday babe! Thank you for being you. Love you

A post shared by Sofia Richie (@sofiarichie) on

It comes after a source claimed Scott and Sofia Richie's romance is getting "more serious" and the couple are "talking about moving in together permanently".

The insider shared: "Scott and Sofia are still going strong and have gotten more serious the past few months. They have definitely had their ups and downs, but lately everything has been really good between them.

"Lionel Richie has finally warmed up and accepted the fact that Scott Disick is dating his daughter Sofia. Lionel had a hard time with the situation at first, and warned Sofia to end the relationship before she got in too deep with Scott, but he now has finally come full circle."

And Sofia has been "great" for Scott and she has made a "big impact" on his life.

A source said: "She's been great for him. She's made a big impact on his life and hasn't partied at all since they met. They've been travelling together to Mexico, New York and Venice and just hanging out. His friends adore her and nobody notices the age difference. She is very mature, she grew up in Hollywood and has always been in older situations. They seem really happy."