What is at stake for Beijing is the recovery of up to US$50 billion in public money and the prosecution of the officials and business people who stole it and took refuge in North America, Europe, Australia and New Zealand, countries with a strong legal system and protection for refugees. Since 1998, 230 corrupt officials have been extradited to China, but more than 1,000 remain abroad as fugitives.

Xu Hong, who led the Ministry of Foreign Affairs team that negotiated the treaty, said it marked a major shift in China's tactics to bring back corrupt officials who had fled abroad, mostly choosing North America or countries in Europe that didn't have extradition treaties with China.

Huang Feng, director of the institute of international criminal law at Beijing Normal University, who used to work in the Ministry of Justice, said China needed a practical and flexible attitude and should consider the advantages against the disadvantages.

'Yes, we're limiting our legal sovereignty but, if we can't repatriate the person, then we can't exercise that sovereignty. By making a concession, we can exercise some or all of it,' he said.

Professor Huang is addressing widespread opposition to the deal within China, where many law enforcement officials and scholars argue that it makes a mockery of Chinese law, by allowing these returnees to receive a lighter sentence than if they had been caught at home and than those already punished for the same offence.

But a western diplomat said the treaty was a step forward. 'Everyone wants to show improvements in human rights in China. If people can see commitments from the Chinese government, they will feel better.'

A second western diplomat said China's use of the death penalty was the major obstacle to western countries' signing an extradition treaty with Beijing. 'Western governments can now consider drawing one up, subject to China providing sufficient evidence against the criminals whom they want to extradite and sufficient guarantees that it will honour its commitments. They would also enable western countries to get back criminals who have fled to China,' she said.

It was by making a concession that Beijing got back its first major economic criminal from the US, in April 2004, when the US sent back Yu Zhendong , former head of a Bank of China branch in Kaiping , Guangdong province, accused of helping embezzle US$485 million from the bank.

Beijing agreed not to execute him and to give him a sentence no longer than the 12 years he had been given by a court in Las Vegas for racketeering. The extradition required the assent of Yu himself, who agreed on condition that his wife and children be allowed to remain in the US, even though she had entered the country illegally.

Last month, a court in Jiangmen , Guangdong sentenced Yu to 12 years in prison for graft and embezzlement of public funds and ordered one million yuan of his assets to be confiscated. The gap of two years between his return and his sentence was in part the result of debate within China's judiciary over the terms of his extradition. But two other ex-managers from the Kaiping branch, Xu Chaofan and Xu Guojun, have rejected similar plea bargains to return to China and serve 25 years and 20 years respectively, saying they do not trust the government and fear they'll be executed.

The two went on trial in February in a court in Las Vegas with three others, accused of conspiracy to defraud the Bank of China of US$485 million.

In September last year, Yu testified against the two Xus in Guangzhou for the Las Vegas court hearing, believed to be the first time the mainland has allowed a US deposition to be taken on its territory. The lawyers for the Xus are challenging the admissibility of Yu's testimony.

The most high-profile criminal whom Beijing wants to repatriate is Lai Changxing , who fled to Vancouver in 1999 and is accused of heading a smuggling ring controlling US$6 billion in Fujian province .

China's ambassador to Ottawa has given a written undertaking that, if he were repatriated, Lai would not be executed. But Lai does not believe him and is fighting extradition in the courts. The Canadian immigration authorities have rejected his application for refugee status but cannot send him back until the end of the legal process, likely to last a few more years.

Asked to comment on the China-Spain extradition treaty, Lai's lawyer, David Matas, said it would have no bearing on his client's case, which had reached the stage of his applying for pre-removal risk assessment. 'He is not losing hope,' he said. 'Mr Lai believes that, whatever Beijing says in public, it may treat those sent back differently. It may execute them, and the outside world has no way of knowing what happens. His brother has already died in prison [in China],' he said.

Another target is Yang Xiuzhu, former deputy director of the construction department of Zhejiang province , who fled to the US in April 2003. In May last year, she was detained at Amsterdam airport, and Beijing has since been negotiating with The Hague for her extradition. It accuses her of stealing more than 250 million yuan in public money.

In March, Zhu Entao, a former assistant minister of public security, said China was confident of the repatriation of Lai, Yang and other high-profile corrupt officials as early as this year. Since 1993, China has signed extradition treaties with 28 countries - mostly developing ones - including Thailand, Laos, Belarus and Brazil. 'Negotiations with developed countries were moving slowly,' said Mr Xu of the Foreign Ministry.

But it's not one-way traffic. Thousands of Chinese have moved illegally to western countries, which want to send them back but find it difficult, for legal, political and diplomatic reasons.

Last month, the head of the US Department of Homeland Security was in Beijing trying to persuade the government to accept 39,000 illegals from the US. China tops the list of foreign countries that refuse to co-operate with repatriation when their citizens are found in the US illegally.

Michael Chertoff said almost 700 Chinese nationals - who each cost the government US$95 a day to detain - were held in US detention centres. He said they were clogging the system and that more than 38,000 had been released on bond after spending the maximum 180 days in lockup.

'Returning Chinese citizens is a major financial burden and low priority for China,' Mr Chertoff said.

Beijing says it is opposed to illegal immigration and is willing to accept illegals back once it's established their real identity, a difficult task since many burn their documents. In practice, with the aid of family networks, underground employment and sophisticated legal advice, thousands of Chinese can stay in Europe and the US illegally.