A judge said care provided by California's prisons had fallen below standard of decency [AP]

The US Supreme Court has ordered California to release tens of thousands of inmates or take other steps to ease overcrowding in its prisons to prevent "needless suffering and death".

The court on Monday told the nation's largest state prison system to sharply cut its inmate population in stages over two years in one of the biggest prison release orders in US  history.

Justice Anthony Kennedy said for the court majority that the medical and mental health care provided by California's prisons had fallen below the standard of decency required by the US Constitution.

Kennedy cited suicidal inmates being held for prolonged periods in telephone booth-sized cages, backlogs of up to 700 prisoners waiting to see a doctor for care and as many as 54 inmates sharing a single toilet.

'Needless suffering'

He said drastic action was needed.

California's 33 adult prisons were designed to hold about 80,000 inmates and now have about 145,000.

In 2009 the state was ordered to reduce the prison population by 46,000 inmates to get down to what judges decided would be a reasonable level. It was the largest prisoner release order ever from a federal court.

Because of prison population reductions while the case has been on appeal, Kennedy calculated the state may have to release 37,000 more inmates or take other measures, such as putting low-risk offenders into community-based programmes and building more prisons.

Improving conditions in California's prisons has become a major legal, political and financial issue in view of the worsening budget crisis in the nation's most populous state.

One of the dissenting justices, Samuel Alito, wrote: "I fear that today's decision, like prior prisoner release orders, will lead to a grim roster of victims."

California had argued that forcing it to release inmates would increase murders and crime.

"As we work to carry out the court's ruling, I will take all steps necessary to protect public safety," California Governor Jerry Brown said in a statement.

The state has been cutting the prison population for years and needs to cut nearly 34,000 more to hit the court-mandated goals. Officials hope they can meet it chiefly by transferring prisoners to local jails.

The dramatic rise in California's prison population had been fuelled by tough sentencing laws adopted during the 1990s.

The United States has more than 2 million people in state and local prisons. It has long had the highest incarceration rate in the world.

Source: Agencies