London, England (CNN) -- China should not execute a British man convicted of smuggling heroin, a top United Nations official said Thursday, days before the execution is scheduled to take place.
Akmal Shaikh, 53, is due to be put to death on Tuesday, having exhausting all his legal appeals. Campaigners say he is mentally ill, and that Beijing did not take that into account when trying him.
Philip Alston, the United Nations' special rapporteur on extrajudicial executions, said it would be a "major step backwards for China" to execute a mentally ill man.
"Both Chinese and international law clearly indicate that a person who committed a crime while suffering from significant mental illness should not be subjected to the death penalty," Alston said in a statement released by Reprieve, a British legal group. "I very much hope that the government will grant clemency in this case."
The British government also has asked China not to execute Shaikh.
But China says it has followed the law.
"This case has always been handled according to law. During the trial, the defendant has been guaranteed his legal rights," Jiang Yu, spokeswoman for China's Ministry of Foreign Affairs, said Tuesday. "Everyone knows that international drug smuggling is a grave crime."
Shaikh was convicted of carrying up to 4 kilograms (almost 9 pounds) of heroin at the Urumqi Airport in September 2007. His final appeal -- to the People's Supreme Court -- was rejected Monday.
He would be the first European Union citizen executed in China in 50 years, Reprieve said.
The organization claims Shaikh may be suffering from bipolar disorder, a severe mental condition characterized by delusional and manic behavior. The group claims Chinese authorities have refused requests for Shaikh to be examined by a doctor and for his mental condition to be taken into account during his trial and sentencing. Watch how Reprieve says Shaikh was duped into carrying drugs
"We deeply regret that mental health concerns had no bearing on the final judgment despite requests by Mr. Shaikh's defense lawyer and repeated calls by the prime minister, ministers, members of the opposition, as well as European Union," the British Foreign Office said earlier this week.
A spokesman for China's Ministry of Foreign Affairs told CNN in October there was no evidence of mental illness.
"The British Embassy and a British organization proposed to have a psychological exam but could not offer any proof of mental illness," the spokesman said. "The defendant himself said that his family does not have a history of mental illness."
Shaikh claimed he was given a suitcase to carry by another man who had duped him into believing he was traveling to China to become a nightclub performer, and he said he was unaware of the drugs concealed inside.
Reprieve campaigners have revealed details of Shaikh's erratic lifestyle prior to his arrest -- including traveling to Poland to start an airline and then on to Central Asia to become a pop star.
While living in Poland, Shaikh was approached by a man who helped him write a song that Shaikh believed would bring world peace, according to Reprieve.
The man said he knew people in Kyrgyzstan who could help Shaikh become a pop star. Once there, Shaikh was introduced to another man called Okole who told him he owned a nightclub in China where they would launch his singing career.
The pair traveled together to Tajikistan, staying in a five-star hotel.
Okole then told Shaikh he would have to travel on to China himself because there was only one seat available on the plane -- and gave him the suitcase to carry, according to Reprieve.
A forensic psychologist said he strongly suspected Shaikh is suffering from a severe mental disorder.
Dr. Peter Schaapveld traveled to Urumqi earlier this year for Shaikh's appeal hearing but was unable to meet Shaikh or attend the appeal. He said British consular staff told him court officials had been "bemused and amused" by Shaikh's "incoherent" testimony.
Schaapveld also examined hundreds of pages of rambling e-mails sent by Shaikh to the British Embassy in Poland and various public figures, including then-U.S. President George W. Bush and singer Paul McCartney.
He said the evidence "very clearly" suggested Shaikh was "probably suffering from bipolar disorder and may also have an additional delusional psychosis."
CNN's Jo Ling Kent in Beijing and Simon Hooper in London contributed to this report.