Britney Spears' boyfriend Sam Asghari celebrates ‘lioness’ singer on her 40th

The personal trainer and actor, 27, shared a touching message from his Instagram account, in which he praised Spears’s “relentless strength” and “beautiful heart”.

“I call you lioness because I admire your relentless strength, I’m inspired by your beautiful heart, I celebrate your smile that lights up my world,” he wrote.

“Everyday is your birthday. my queen. Happy first birthday to my wife.”

Asghari and Spears met in October 2016 on the set of her “Slumber Party” music video, in which he starred as her love interest.

Spears announced their engagement in September this year, in a video posted to Instagram.

In the video the “Toxic” singer is seen showing off a four-carat diamond ring engraved with the word “lioness” – Asghari’s nickname for Spears.

It also shows Spears winking, kissing and smiling at her fiancé and answering “Yes!” when he asks if she likes the ring.

The 39-year-old captioned Sunday night’s post: “I can’t f***ing believe it.”

Asghari has been praised for providing support for Spears during her conservatorship battle, which ended when the legal arrangement was officially terminated earlier this month.