Williams co-founded the notorious Crips gang

Williams, 51, will be executed on Tuesday unless Governor Arnold Schwarzenegger grants clemency.

Williams' defence team say a new witness has offered fresh evidence which could exonerate him. But a fresh appeal was rejected on Monday.

A co-founder of the notorious Crips street gang, Williams denies murdering four people in 1979.

His supporters range from Oscar-winning actor Jamie Foxx and rap star Snoop Dogg (himself a former Crips member) to Bishop Desmond Tutu and the Reverend Jesse Jackson.

During his 24 years in jail, Williams has won praise for his anti-gang books, earning Nobel Peace Prize nominations for his teachings.

But relatives of some of his victims have insisted he does not deserve clemency.

'Agonising decision'

Defence lawyer Jan Handzlik told CNN on Monday that fresh information obtained from a new witness had been sent to the California governor.

Governor Arnold Schwarzenegger is considering a request for clemency

On Sunday, California's Supreme Court refused to grant a stay of execution, ruling that claims made by Williams' defence had no merit.

The defence ask a federal appeals court to review the case. But the US 9th Circuit Court of Appeals, which had declined to reconsider the verdict in February, rejected the move on Monday.

Mr Schwarzenegger held a clemency hearing on Thursday and has said he is agonising over the decision.

Governor Arnold Schwarzenegger has until midnight (0800 GMT) to decide whether to grant a clemency request.

No clemency request has been granted in the state since 1967.