Pakistan is preparing to execute a man convicted over a murder committed when he was 14, in a move likely to provoke a storm of international anger.

A juvenile cannot face the death penalty in Pakistan, but a so-called “black warrant” ordering the imminent hanging of Shafqat Hussain was announced suddenly last week, overturning government pledges to examine his case and age – in defiance of an international outcry and allegations that the boy was innocent and tortured into a confession.

On Monday lawyers for Hussain will file papers seeking to stay the execution, due on Thursday, until allegations of torture have been heard and Hussain’s age investigated – as promised by Pakistan’s interior minister, Chaudhry Nisar Ali Khan, who suspended the sentence in January.

“We have his birth certificate,” Hussain’s lawyer, Sarah Belal, told the Observer. “The government stated that the execution would be stayed so there could be an inquiry into his age. There has been none; no one has contacted Shafqat or us, and we hold the family papers. Our case worker visited Shafqat on Friday and he was adamant he had not been asked about anything. So why not, and why is this juvenile suddenly to be killed?”

Hussain has been on death row for 10 years. The hanging would be the second to follow a lifting last week of Pakistan’s moratorium on executions for crimes other than terrorist offences. A moratorium on the execution of terrorists was discarded in December after Taliban gunmen killed 150 people at a school in Peshawar.

“It’s beyond grotesque,” said Clive Stafford Smith, founder and director of Reprieve, the world’s leading anti-death penalty group. “If they have to kill people, why not those who mass murder pupils in a school? Why an innocuous juvenile who has been tortured into confessing a crime he did not commit? It reflects total dysfunction, and reneges completely on what the government promised only a few weeks ago.”

Belal said: “There are many in the federal and provincial bureaucracy opposed to a death penalty moratorium of any kind. They think it’s un-Islamic and they should all be killed.” Her young client was, she said, “lumped in with all that – a scapegoat they want to hang along with a multiple murderer on the same day, same jail”.

Hussain, from the poor Neelam valley, went to Karachi aged 13 to join his brother, Manzoor, and work as a caretaker. He was arrested a year later, after the disappearance of a tenant in the building following a dispute and, said Belal, illegally held, bound, blindfolded and beaten. After nine days of torture, Hussain signed a confession and was tried, for reasons unknown, in a special anti-terrorist court where an appointed lawyer failed to ascertain his age or produce any defence witnesses.

Facebook Twitter Pinterest Clive Stafford Smith has condemned Pakistan’s move to executive Shafqat Hussain. Photograph: Graeme Robertson

Hussain was sentenced to death in 2004 and lost an appeal in 2007. The sentence is now back in force after the 12 March announcement lifting death row moratoriums for all crimes, a move condemned by Human Rights Watch as an “ill-conceived decision” opening the way for “an execution spree”. Pakistan has more than 8,000 prisoners on death row.

When the initial moratorium saga unfolded in December, lawyers from Justice Project Pakistan challenged Hussain’s sentence in the high court on the ground that he was tortured and because of his age when he was alleged to have committed the crime. The court ruled it could not quash a sentence delivered by the supreme court, but Nisar halted Hussain’s execution, pledging the establishment of an inquiry, and told a press conference that DNA tests would be done to verify it.

Nisar told parliament on 5 January: “Members of this house brought the case of a boy, Shafqat Hussain, to the attention of the ministry and me. Mr speaker, we halted his execution, it is pending for now, and there has also been an inquiry ordered on that. Because whatever means you use to take someone’s life, you are answerable.”

But suddenly, on the day the moratorium for non-terrorist crimes was abandoned, an order was issued for Hussain’s execution to proceed a week later, although DNA had not been tested and Hussain’s family not been contacted about what Stafford Smith calls the “abundant evidence of his birth date”.

Belal said: “Our case in the high court on Monday does not address the legal reasons why it could not reopen the question of juvenility. We are saying we have a legitimate expectation of an inquiry, from the minister himself.” On torture, she added: “I’m afraid torture by the police is so endemic and widespread, the criminal justice system has become desensitised to it.”

Reprieve, which is based in Britain, is seeking to coordinate international efforts in a three-day scramble to save Hussain’s life. Stafford Smith – known for his work in America and his book on executions after miscarriages of justice there – hopes to secure a European Union presence at Monday’s hearing. “It is never too late,” he said, “to prove you are innocent and the victim of torture. Even if Shafqat did do it, as a juvenile he can’t be executed.

“They’ve had 11 years to ascertain his age, and the fact that they haven’t reflects total incompetence,” Stafford Smith said. “How can you expect to assess his guilt or innocence of a crime if you cannot even figure out his age? We’ve always argued that the problem with the death penalty is human failure, that you cannot kill people when there’s so much uncertainty. When I was in Pakistan recently I asked a judge what level of certainty he needed to deliver a death sentence. He replied: ‘About 60%’. It’s appalling, it’s madness.”

The writer and rights campaigner Fatima Bhutto, granddaughter of the Pakistani statesman Zulfikar Ali Bhutto, says: “At the same time the government was fast-tracking the execution of a boy tortured and convicted at the age of 14, the Islamabad high court ordered the release of Zaki ur Rehman Lakhvi, the accused mastermind of the Mumbai terror attacks. There is no justice here.”

Shafqat’s brother Manzoor said: “We are very poor people and we cannot buy justice like the powerful. The government said he should not be hanged when the international community and media were paying attention, but now they are not concerned. The government is only interested in showing they are doing something about terrorism. But my brother is not a terrorist.”