Capital punishment for juveniles had been allowed in 19 states

The court was divided on the issue, but voted 5-4 that the death penalty for criminals aged 16 and 17 should be declared unconstitutional.

The decision affects not only those convicted in future, but about 70 prisoners already on death row for offences committed before they were 18.

Anti-capital punishment campaigners claimed the decision as a victory.

"Now the US can proudly remove its name from the embarrassing list of human rights violators - that includes China, Iran, and Pakistan - that still execute juvenile offenders," said William Schulz, executive director of Amnesty International USA.

'No deterrent'

The highest US court upheld an earlier ruling by the Missouri Supreme Court, which banned the execution of people convicted of crimes they committed before turning 18.

The Missouri court said putting minors to death was a violation of the US constitution, which outlaws "cruel and unusual" punishment.

JUVENILE EXECUTION 19 states allow execution - only six have carried it out 227 juveniles sentenced to death since 1976 22 of them executed - 13 in Texas More than 3,400 people on death row in total

That ruling overturned the death sentence given to Christopher Simmons, who was 17 when he kidnapped a neighbour, tied her up and threw her from a bridge to her death.

Simmons's attorney, Seth Waxman, said the death penalty did not deter minors, since "they weigh risks differently" to adults.

There are 19 states where capital punishment for juveniles is allowed.

But Supreme Court Justice Anthony Kennedy, who voted for the ban, noted that even in these states the ultimate sanction was not often carried out.

He said the trend was to abolish the practice.

"Our society views juveniles... as categorically less culpable than the average criminal," he wrote.

Christopher Simmons will not now be executed

"The age of 18 is the point where society draws the line for many purposes between childhood and adulthood. It is, we conclude, the age at which the line for death eligibility ought to rest."

One of the court's dissenting judges, Sandra Day O'Connor, argued that: "Chronological age is not an unfailing measure of psychological development, and common experience suggests that many 17-year-olds are more mature than the average young 'adult'."

Tuesday's ruling follows several others that have limited the use of capital punishment.

In 1988, it was ruled that offenders who were younger than 16 when they committed their crimes could not be executed.

In 2002, the court banned the killing of offenders with mental disabilities.

Former US President Jimmy Carter and many foreign governments, including those of the European Union, were among those who had called on the US to end the juvenile death penalty.

Judge Kennedy said: "It is proper that we acknowledge the overwhelming weight of international opinion against the juvenile death penalty, resting in large part on the understanding that the instability and emotional imbalance of young people may often be a factor in the crime."