California prison authorities have warned thousands of hunger-striking inmates they could face discipline for illegal "mass disturbances".
Authorities confirmed that more than 12,000 prisoners missed nine consecutive meals in the past three days.
The hunger strikers, who are also refusing work assignments, are protesting what prisoner advocates describe as the state's inhumane solitary confinement practices.
These can include locking inmates in isolated cells for up to 23 hours a day.
The action launched on Monday by inmates at over two dozen correctional facilities marked the largest prison hunger strike in California history, according to the Los Angeles Times.
It is nearly twice the size of a 2011 strike that at its peak involved 6,500 inmates.
"Participating in a mass disturbance and refusing to participate in a work assignment are violations of state law, and any participating inmates will receive disciplinary action," state corrections officials said in a statement in response to the strike.
Joining in this current strike "can lead to loss of privileges, loss of credits," said Jeffrey Callison, a spokesman for the Department of Corrections and Rehabilitation.
He did not say what types of inmate privileges could be taken away.
The department said the strike was led by prison gangs and that "mass hunger strikes, work stoppages and other disruptions" could potentially affect safety and security behind bars.
The strike comes at an already challenging time for the prison system in the most populous US state, which has been ordered by a federal court to reduce prison size by 10,000 inmates this year to ease crowding.
The state has begun housing many low-level prisoners in county jails. Governor Jerry Brown has been at odds with federal judges over demands the state continue to reduce inmate numbers.
California holds 4,527 inmates isolated from the general prison population in so-called Security Housing Units because prison officials found they had committed crimes behind bars or have gang affiliations, a Department of Corrections and Rehabilitation spokeswoman said.
The Prison Hunger Strike Solidarity group said on its website yesterday that about 30,000 inmates were participating, a figure higher than numbers cited by officials.
Corrections officials had said that more than 30,000 of California's 132,800 inmates began refusing food on Monday and continued to decline meals through the week, with nearly 29,000 participating on Wednesday.
Yesterday, officials said 12,421 prisoners in 24 state prisons and four out-of-state facilities had missed nine straight meals, a benchmark required for officials to recognise in a hunger strike.
They did not say whether any other prisoners may have been participating but had missed fewer meals.