His appeal was held in secret and without his knowledge. With legal avenues now exhausted, Ali could be executed at any moment.
A teenager arrested for taking part in an anti-government protest in Saudi Arabia is set to be crucified and beheaded after his appeal was dismissed.
Campaigners are losing hope that Saudi King Salman bin Abdulaziz will grant him a pardon.
Ali Mohammed al-Nimr was arrested in 2012 aged 17 and has been sentenced to die by beheading and crucifixion after a closed trial on charges of encouraging pro-democracy protests using his BlackBerry.
He was also accused of illegally possessing firearms, a charge he strongly denies, reports the Daily Mirror.
Critics say he is innocent and is being punished because his uncle is a prominent religious leader and human rights activist, Sheikh Nimr al-Nimr - who was also sentenced to death in 2014.
They go on to claim the teenager was tortured, denied access to a lawyer and not even told when his case was taking place until after he was sentenced to death. They say he was forced to signed a confession and that there is no evidence against him.
Campaigners are now calling on the UK Government to put pressure on its allies in Saudi Arabia to block the execution.
Maya Foa, director of the death penalty team at legal charity Reprieve said: "Ali was a vulnerable child when he was arrested and this ordeal began.
"It is hard to see what British interests are strong enough to trump the principle that we should not be supporting the 'crucifixion' of juveniles."