China - British Prime Minister Gordon Brown said he was appalled at China's execution of a Briton caught smuggling heroin on Tuesday, prompting China to denounce British accusations and defend its court system.

Relatives of Akmal Shaikh, 53, and the British government had appealed for clemency, arguing the former businessman suffered from bipolar disorder, or manic depression. China's Supreme Court rejected the appeal, saying there was insufficient evidence of mental illness.

Brown condemned the execution, carried out in the far-west region of Xinjiang, in strong words that may raise diplomatic temperatures over the case.

I condemn the execution of Akmal Shaikh in the strongest terms, and am appalled and disappointed that our persistent requests for clemency have not been granted, he said in a statement issued by the British Foreign Office.

I am particularly concerned that no mental health assessment was undertaken.

China was just as determined in its defense of the execution.

Nobody has the right to speak ill of China's judicial sovereignty, Chinese Foreign Ministry spokeswoman Jiang Yu said. We express our strong dissatisfaction and resolute opposition over the groundless British accusations.

Shaikh was executed by injection, China's official Xinhua news agency reported. His family said it was stunned and disappointed and criticized China's stance on his mental health.

We are astonished at suggestions that Akmal himself should have provided evidence of his own fragile state of mind, they said in a statement.

Shaikh was the first European citizen to be executed in China since 1951, Western rights groups say. China executes more people than any other country, with about 1,718 executions in 2008, far surpassing Iran at 346 and the United States at 111, according to Amnesty International. China does not release an official count of its executions.

The case could harden public opinion in Britain against China. It could also rile Chinese resentment over what Beijing often calls interference in its internal affairs, mindful of humiliating defeats by Britain during the Opium Wars of the 1800s.

We hope that the British side can view this matter rationally, and not create new obstacles in bilateral relations, Jiang said.

Britain is China's third-largest trade partner in Europe, with total trade of $45 billion in 2008. The two nations recently traded jibes over the troubled Copenhagen climate change negotiations.

Heroin use is a major problem in Xinjiang, which borders Central Asia. The region was convulsed by ethnic violence and protests in July, with further protests in September after widespread panic over alleged syringe attacks.

All executions in Urumqi have used lethal injections in recent years, said a detention center official surnamed Jia.

Shaikh's defenders, including British rights group Reprieve which lobbies against the death penalty, say he was duped into smuggling heroin by a gang who promised to make him a pop star.

Arrested in 2007, a Chinese court rejected his last appeal on December 21. Reprieve posted on the Internet a recording Shaikh made of a song, Come Little Rabbit, which Shaikh believed would be an international hit and help bring about world peace.

This is not about how much we hate the drug trade. Britain as well as China are completely committed to take it on, British Foreign Secretary David Miliband said in a statement. The issue is whether Mr Shaikh has become an additional victim of it.

(Additional reporting by K.J. Kwon; Writing by Lucy Hornby and Chris Buckley; Editing by Nick Macfie)