Forty years after the great civil rights battles in the American South, one of the last bastions of formal racial segregation in the United States is set to topple, following a Supreme Court ruling decrying the California prison system's practice of separating black, Latino and white inmates.

The nation's highest court said the principle at stake was the same that led to a landmark ruling in 1954 ordering school desegregation - the idea that there is no way to separate people and say meaningfully that they still enjoy equal rights under the law. "We rejected the notion that separate can ever be equal ... 50 years ago in Brown vs Board of Education," Justice Sandra Day O'Connor said in the majority ruling, "and we refuse to resurrect it today."

The Supreme Court stopped short of declaring prison segregation to be unconstitutional, referring the case back to the federal appeals court. But the 25-year-old Californian policy of sorting inmates by race on admission to the prison system is almost certainly doomed - only the timing remains in doubt.

The case carries considerable significance since California has by far the largest state prison system in the US, with some 160,000 inmates, and has also grappled for the past 40 years with an explosion in violent, racially based prison gangs.

State prison officials argue that because the existence of gangs such as the Aryan Brotherhood, the Mexican Mafia and the Black Guerrilla Family, many of whose members they say would rape, assault or kill a member of a different race given half a chance, keeping them apart is the only way to prevent anarchy.

Proponents of desegregation, however, say race-based separation exacerbates the problem and deepens the inter-racial animus. "Indeed, evidence demonstrates that integrating inmates reduces all forms of prison violence," Senator Gloria Romero, who has been leading the anti-segregation charge, wrote recently.

Bert Deixler, the lawyer who took the case to the Supreme Court on behalf of a black inmate appalled by the blanket discrimination he found on entering prison, argued that the California policy was discriminatory and wrong-headed, since it relied on racial stereotyping to make certain assumptions about inmates.

"You can look at gang membership as a basis for special treatment," he said, "but you can't look at people coming off the bus and say, 'Blacks go through that door and whites go through the other door'. This policy assumes if you are of a certain race, you have a penchant for interracial violence."

Nobody is underestimating the bewildering problem of California's prison gang culture, which is a reflection of and a breeding ground for street gangs in Los Angeles, Oakland and elsewhere. The Mexican Mafia, also known as La Eme, dates back to the 1950s, and is now a thriving presence on the streets as well as behind bars, involved in drug trafficking, money-laundering, prostitution and many other rackets.

Its main rival is another Mexican gang called Nuestra Familia, based in northern California. Although vendettas between the two are common, there is little or no evidence that either Latino gang holds grievances against other racial groups.

Of particular concern to prison officials was the creation of the Aryan Brotherhood in Folsom State Prison in the late 1960s, because anti-black violence was part of the gang's ideology from the start - including a reputed stipulation that killing a black man was the passport to entry into the gang.

Since then, California's increasingly overcrowded maximum security facilities have been racked by spasms of violence, including a fight that broke out 18 months ago in a private facility where black, Latino and white inmates hacked at each other with knives and meat cleavers stolen from the kitchen.

Last year, riots broke out in several prisons, including one jail where a Latino guard was stabbed to death by gang-affiliated inmates.