Britney Spears and Sam Asghari engaged: Pop star shows off rock on Instagram

Britney Spears and Sam Asghari have announced their engagement on Instagram. 

The 'Toxic' singer showed off a diamond ring to her 33.9 million followers in a video captioned: "I can't f**king believe it!" 

In the short clip, Spears' fiance Asghari says: "Look at that, do you like it?" prompting the Grammy winner to reply: "Yes!" 

The pair have been dating for nearly five years, having met on the set of Spears' music video for 'Slumber Party' in 2016, where Asghari played the singer's love interest.

It will be the pop star's third marriage, having wed longtime friend Jason Alexander in Las Vegas in 2004 before a quick annulment, then marrying Kevin Federline that same year. 

A statement given to Page Six by Asghari's talent manager confirmed the news. 

"The couple made their long-standing relationship official today and are deeply touched by the support, dedication and love expressed to them," the statement read. 

"Roman Malayev from NYC is the ring designer. He couldn't be more happier [sic] to be involved and make this a one-of-a-kind ring." 

The four-carat stone reportedly sits on a band with Asghari's nickname for Spears - 'lioness' - engraved inside it. 

"Every designer I spoke with about the ring was amazing with great ideas, but I just clicked with Roman - we really connected on the design and he was as excited as I was to make it special - that's why I chose him," Asghari said in a statement. 

Earlier this year, Spears told the Los Angeles Superior court that she was prevented from having another baby or getting married as part of the alleged "abuse" taking place under the conservatorship helmed by her father Jamie Spears. 

"I have an [IUD] in my body right now that won't let me have a baby and my conservators won’t let me go to the doctor to take it out" she said in an emotional testimony. "I feel ganged up on. I feel bullied and I feel left out and alone." 

"All I want is to own my money and for this to end and for my boyfriend to be able to drive me in his f**king car."