Britney Spears' boyfriend Sam Asghari has taken aim at the pop superstar's father in a fiery Instagram post, blasting him for "trying to control" the couple's relationship.

Asghari has been by the singer's side ever since meeting on the set of her 2016 music video for Slumber Party, and he has been supporting the Toxic hitmaker as she fights to have her dad, Jamie Spears, permanently removed as co-conservator of her estate.

Following the release of a new documentary, Framing Britney Spears, which delves into the musician's rise to fame, as well as her subsequent mental health and legal struggles, and her conservatorship, Asghari insisted he's always strived to give Britney the future she "wants and deserves", telling "I am looking forward to a normal, amazing future together."

However, on Tuesday, Asghari, who rarely goes into detail about the couple's romance, unleashed an online attack on his girlfriend's father, claiming he has been attempting to sabotage their relationship.

"Now it's important for people to understand that I have zero respect for someone trying to control our relationship and constantly throwing obstacles our way," he wrote on his Instagram Story timeline.

"In my opinion Jamie is a total d**k," he slammed.

The Iranian-born actor added: "I won't be going into details because I've always respected our privacy but at the same time I didn't come to this country to not be able to express my opinion and freedom."

Following his social media outburst, Asghari told TMZ he was standing by his remarks about Jamie, although he hopes the issues can be resolved.

"Once he starts treating his daughter right, then we can be on good terms," he said, before assuring fans Britney is "doing amazing" amid the conservatorship battle.

Britney has been under the conservatorship ever since her public meltdown in 2008, and despite her efforts to have Jamie removed from the agreement, a court official recently decided to keep the legal order in place until September.