Lawsuit claims Bobbi Kristina's boyfriend put her face down in bath

The family of Bobbi Kristina Brown are said to have filed a lawsuit against her boyfriend claiming he gave her a "toxic cocktail" before putting her face down in a bath tub.

They accuse Nick Gordon of the "wrongful death" of Bobbi Kristina, who suffered irreversible brain damage in January, according to US reports.

She never regained consciousness and died in hospice care last month.

Her death bore striking similarities to that of her mother Whitney Houston who drowned after suffering a heart attack linked to cocaine abuse in 2012.

Whitney Houston and her only daughter Bobbi Kristina died three years apart. Credit: Reuters

The lawsuit alleges that on the day of the bath tub incident Gordon and Bobbi argued for about 30 minutes before "everything became abruptly quiet".

According to the suit, Gordon then "gave Bobbi Kristina a toxic cocktail rendering her unconscious and then put her face down in a tub of cold water causing her to suffer brain damage".

It is alleged Gordon then changed his clothes before coming back downstairs where they had guests.

The alarm was not raised until 15 minutes later when a guest went to check on Bobbi Kristina.

A dust pan was allegedly at the bottom of the tub and the 22-year-old was unresponsive with a tooth hanging loosely from her mouth.

Bobbi Kristina as a child with mum Whitney and dad R&B singer Bobby Brown. Credit: Reuters

Bobbi was rushed to hospital where she was revived and put in a medically-induced coma.

The lawsuit also alleges Gordon was abusive to Bobbi Kristina in the lead up to her death.

Several witnesses are said to have seen Gordon punch her as she sat on a sofa days before the bath tub incident.

The impact was so great, the suit claims, that the sofa broke knocking Bobbi to the floor and Gordon continued to attack her "hitting her in the face until she was bloody".

Gordon then allegedly dragged Bobbi upstairs by her hair later telling witnesses: "I don't do this often."

The lawsuit asks for damages of at least $10 million each for the assault, battery, emotional distress and allegedly transferring money from Bobbi's bank account into his own without permission.

No criminal charges have been filed against Gordon.