The Douglases' solicitor, Louise Christian, said last night she would seek a High Court judicial review of the verdict at Southwark Crown Court, south London.
It would be on the grounds that the presiding coroner Sir Montague Levine erred in law when he instructed the jury about the conditions necessary for a verdict of unlawful killing.
Ms Christian said: "The family are extremely disappointed at this verdict and on behalf of Lisa and Albert Douglas [sister and brother of Wayne], I would like to express that disappointment. There was no reason for him to die in Brixton police station."
She added: "Time and time again, people - particularly black people - are dying in police cells and no action is taken. We will be going to the High Court to challenge this verdict because the coroner erred in law."
The majority decision provoked a walkout by many of the former postman's 20 or so relatives and friends present in court.
It also prompted Sir Montague to make a series of recommendations regarding police training and procedures. The four-man, five-woman jury said that in their opinion Mr Douglas, 25, died from "left ventricular [heart]" failure.
They said this was caused by stress and exhaustion, as well as positional "asphyxia" - lying face down with hands cuffed behind him long enough to cause fatal breathing problems.
They linked their eight-to-one conclusion to a "chase, and series of restraints in the prone position face down as used by current police methods". The coroner said the effects of restraining a suspect, especially in the face-down position, needed more research and evaluation.
Restraint and its effects following periods of exhaustion and stress needed to be the subject of specific guidelines for all officers, who should have a booklet on the subject plus receive refresher courses.
Mr Douglas's brother Albert, 39, said he was bitterly disappointed, claiming his brother was suffocated. "That is what happened to him. That is how he died. He was suffocated in Brixton police station."
Mr Douglas's girlfriend Nadine Beckford, a 20-year-old student from Brixton who went out with him for three years, had said earlier that she was hoping for a "good verdict" - unlawful killing.
She said last night: "I'm surprised and unhappy about it. I don't know how they came to that verdict. Maybe one person got an idea and persuaded everyone to go with it."
The inquest heard that the events leading up to Mr Douglas's death in the early hours of 5 December last year began with a robbery at knifepoint of a couple in their home in Brixton, south-west London.
Within minutes, Mr Douglas, who answered the intruder's description, was challenged by police in a nearby street.
He produced an 8in kitchen knife, similar to one used in the burglary and a stop-start chase followed in which Mr Douglas, of Crystal Palace Park Road, Sydenham, south-east London, repeatedly lunged at the officers.
They finally cornered him, striking him three times with their long-handled batons to disarm him. He was then wrestled to the ground, handcuffed and carried to a police van.
Some residents spoke of officers pouncing on the man after he had voluntarily dropped the knife, and subjecting him to a prolonged beating as he screamed in agony.
Each of the three medical experts the inquest heard from said the grazes and bruises Mr Douglas suffered bore no relation to the eyewitnesses' graphic accounts. Furthermore, none of his physical injuries played any part in his death.
Two of the experts, both pathologists, went on to say that they had no doubt the various occasions when Mr Douglas was in a face-down position had an accumulative affect in weakening his otherwise normal heart and leading to his death.
Within 20 minutes of arriving at Brixton police station he was dead.
The death led to furious rioting in Brixton.Reuse content