Media playback is unsupported on your device Media caption China's citizens are sending their money overseas at an unprecedented rate

In the shadow of Hong Kong's big banks, rows of currency exchange shops specialise in quick, anonymous transactions.

But behind the scenes, much larger deals are helping to move money at an unprecedented rate. Wealth is flowing from the mainland, through currency dealers in Hong Kong and beyond.

The leaked Mossack Fonseca documents have revealed to us how the families of China's leaders keep money offshore.

And now, a full analysis of the files by the International Consortium of Investigative Journalists shows that nearly one third of the firm's business came from its offices in Hong Kong and China - making China the firm's biggest market and Hong Kong the company's busiest office.

Destabilising the economy?

Mossack Fonseca's booming China business is evidence of an even bigger trend: the reliance of China's wealthiest people on offshore investments.

Around $1tn (£700bn) left China last year, draining the country's foreign reserves.

It is a shift that could destabilise the entire Chinese economy.

And the relatives of China's leaders are among those who have stashed their wealth abroad.

At least seven current and former leaders were found to have links to offshore companies set up by the Panamanian law firm, including the Chinese President, Xi Jinping, and two other top leaders.

Many of these names have circulated in connection with offshore banking before, in past media reports. However, the leaked files come at a tricky time for China's leadership.

Owning offshore companies is not illegal in China but the existence of these secretive financial structures raises all sorts of questions for the families of China's leaders.

China's Communist officials are supposed to lead "clean" lifestyles that discourage them from profiting from their ruling positions, according to the party's constitution. And importantly, their families are not supposed to profit from their ties to the top.

Image copyright AFP Image caption Hong Kong's tourist areas are filled with currency exchange shops

Willy Lam, a political analyst with the Chinese University of Hong Kong, says Xi Jinping has portrayed himself as "a purist in terms of morality and frugality".

Stashing vast sums in offshore accounts "definitely goes against the teachings of Xi Jinping and also well-known conventions of the Communist Party", he says.

"As to whether the offspring of senior cadres have obtained their wealth illegally, it's difficult to say because the Chinese legal system is very opaque."

Capital flight

The files tell us more than we ever knew before about how the Chinese elite's money is stored abroad. Long email chains reveal that Mossack Fonseca repeatedly helped politically connected clients to become offshore company shareholders without exploring their backgrounds, as they are required to do under international law.

For example, Mossack Fonseca helped Deng Jiagui, the brother-in-law of Chinese President Xi Jinping, to create three offshore companies located in the British Virgin Islands.

However, the firm failed to investigate Mr Deng's high-profile political connections when helping him to acquire his companies, in 2004 and again in 2009.

It is unclear what the companies were used for though one had been dissolved and the other two were dormant by the time Mr Deng's powerful relative, Xi Jinping, took the helm of the Communist Party in 2012.

But the irony cannot be overlooked: since coming to power, Xi Jinping has unleashed an intense anti-corruption campaign on the Communist Party. More than 300,000 officials were punished for violating the party's anti-corruption laws in 2015 alone.

Image copyright AFP/Getty Images Image caption Most of the money leaving China is said to be flowing through Hong Kong

What is happening at Mossack Fonseca is being replicated elsewhere too. Rich Chinese are using Hong Kong as a gateway - to protect their wealth by moving it overseas.

"People are worried about keeping their money in China for two reasons," explains Andrew Collier, an independent China analyst based in Hong Kong.

"One is that the Chinese economy is slowing. The second reason is that the leadership has been trying to clean up corruption and there's some sense that some people are trying to move their money offshore because they're worried about the safety of their capital within China."

Hong Kong has become a focal point for those who want to stop money leaving China. Last month, China's anti-corruption office acknowledged that most money flowed through Hong Kong and vowed to stop the practice, though that might be an impossible task.

'There would be panic'

Around $600bn of the money that left China last year was transferred in defiance of Chinese banking controls.

Every Chinese citizen can only transfer $50,000 a year outside the country. Anything more than that is often moved illegally.

Some people use complex money transfers to get their cash out.

Image copyright AFP Image caption An estimated $1 trillion (£700bn) was taken out of China last year, weakening the economy

One illegal currency changer we spoke to explained how he helped clients secretly transfer money abroad by keeping vast reserves of money in dozens of "zombie" accounts spread across China, Hong Kong, Vietnam and the Philippines.

He uses bank accounts that are still under the names of dead people to ensure they cannot be traced back to him.

"I receive my client's money into one account in one country and then transfer the currency they need into a different account in another country," he explains, smiling.

However, he says he will not accept any more business from clients who want to take Renminbi out of China.

"I have too much Renminbi already," he says, frowning.

What would happen if China cracked down on currency traders like him and actually enforced their own rules more tightly?

"Panic. There would be panic."

Money mules

The movement of capital is fuelled by anxiety.

"People do not have faith in the ability of the financial and economic decision-making team to put things right," Willy Lam explains.

"So if they have one or two million US dollars, it would be stupid for them not to at least park half of that wealth overseas. Simply because there is very little faith in the future of the party."

Those who cannot access big-time currency traders sometimes rely on money mules to carry thick stacks of cash across the border. We met one man who works as a mule, who confirmed he was busy carrying money for his anxious clients.

"If my customers want to immigrate or invest in a business overseas, they need my help," he explained.

"Sometimes I strap the money on to my body or I carry a small bag. Customs officers always target people with lots of luggage or those who look nervous, so I just try to act normal."

Image copyright Reuters Image caption Chinese customers make up the world's largest portion of consumers of luxury goods

So why does it matter if China's richest people move their wealth out of the country?

Once money leaves China, it has to go somewhere.

This massive flow of money is driving up real estate prices worldwide. According to juwai.com, a real estate website that connects mainland Chinese clients with foreign sellers, Chinese buyers spent more than $52bn on foreign property last year.

Media playback is unsupported on your device Media caption Juwai.com explains where Chinese are buying property

In Hong Kong, visitors from mainland China splash out on luxury items. This is replicated around the globe: China's richest people - perhaps even those at the very top - are spending and storing their money elsewhere.

They are protecting themselves but they are making China more vulnerable.

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