Image copyright AFP Image caption Many fear that corruption on mainland China has crept into Hong Kong's culture

In East Asia, Singapore and Hong Kong are ranked the most corruption-free territories. Both used to be British colonies, but while Singapore is now an independent country, Hong Kong is a self-governing part of China.

Hong Kong has a reputation as a "clean" city. In Transparency International's 2012 corruption perception index, it ranked 14, above Japan, the UK and the US, with China down at 80.

Since the handover to China in 1997, more interaction with the mainland has created both opportunities to spread the city's experience fighting corruption across China and threats to its clean image.

Hong Kong's anti-corruption organisation, the Independent Commission Against Corruption (ICAC), was forced to investigate its own former chief after an audit revealed that two dinners he hosted overspent the hospitality limit of HK$450 ($58; £38) per person.

The incident took place under Timothy Tong, who stepped down as the ICAC's commissioner in July 2012. More details then emerged of how Mr Tong had spent public money on other banquets and gifts, including for Cao Jianming, China's procurator-general.

To make things worse, Mr Tong was appointed to China's top political advisory body, the Chinese People's Political Consultative Conference, after he retired from the ICAC.

Critics accused Mr Tong of spending public money for his own political prospects, something Mr Tong strongly denies.

He is now under criminal investigation by the ICAC and the scandal has led many to ask whether China's corruption culture is infiltrating Hong Kong.

"Public money"

Image copyright AP Image caption Timothy Tong denies allegations that he spent public money on hospitality for his personal benefit

Hong Kong's anti-corruption revolution began 40 years ago, in June 1973, after Peter Fitzroy Godber, a senior police officer, avoided an investigation into his assets by fleeing to the UK.

His escape ignited an outcry. People held protests demanding his extradition, and relations between Hong Kong's public and the British administration deteriorated.

To save the establishment's credibility, Governor Sir Murray MacLehose announced the creation of the ICAC. The commission opened its doors in 1974 and eventually brought Godber back to Hong Kong, where he served four years in jail.

Since then, the ICAC has played a crucial role.

Barrister Stephen Char, who worked there for 28 years, says the ICAC achieved success by enforcing the law, putting prevention measures in place and conducting public education in parallel.

Several overseas governments, as well as members of the Beijing government, have turned to the ICAC for advice.

ICAC figures seen by the BBC show that around 4,000 mainland Chinese officials visit Hong Kong each year to learn from the anti-corruption body.

Beijing is very concerned and they don't want to see a downfall of the ICAC Stephen Char, Former ICAC officer

However, some of the dinners that caused the Timothy Tong scandal took place during an anti-corruption exchange with the mainland - sparking fears mainland habits had influenced local behaviour.

Prof Tsao King-kwun from the Chinese University of Hong Kong, who specialises in anti-corruption measures, believes that part of the problem is China's traditional culture.

"Conventionally, Chinese officials may think: 'You are a guest, so I have to give you the best hospitality, feed you well, make you feel comfortable.' This is politeness," Prof Tsao explained.

"They don't have the Western creed that business is business - that this is public money and you shouldn't spend it on the wrong thing," he said.

Stephen Char said despite the Timothy Tong case he had faith in the system.

"So many commissioners come by, and he is the only one who got into trouble," he says.

"You may ask if it was cultural assimilation, but let me put it this way: if he had been more self-disciplined, no problem would have occurred."

Mr Char also believes Hong Kong's Audit Commission is serving as an effective check to power, as demonstrated in the Timothy Tong case.

"Firewall"

There have been other examples outside official bodies. In 2006, for example, a City University of Hong Kong professor was offered HK$10,000 ($1289; £865) by a mainland student for answers to a written examination. The professor (who was also from the mainland) turned her in to the ICAC.

ICAC's three major arms Operations Department investigates corruption allegations

investigates corruption allegations Corruption Prevention Department examines practices and procedures of government departments and public bodies to reduce opportunities for corruption

examines practices and procedures of government departments and public bodies to reduce opportunities for corruption Community Relations Department educates the public against corruption Source: ICAC website

More recently, a cross-border trader and three store supervisors were imprisoned and fined over the illegal retention of cans of baby formula milk powder for export to the mainland, where demand is high, after an investigation involving the ICAC.

Prof Tsao says part of the problem is the speed that integration with the mainland has taken place.

"But the key is whether we have a solid ground, a 'firewall' to fend off the chill," he said.

Of course, the city is not completely free of home-grown corruption either.

In July 2012, Hong Kong's development secretary Mak Chai-kwong was forced to resign less than two weeks after he began his term, after he was arrested for housing allowances fraud. He has since been convicted.

But that case notwithstanding, the future of the ICAC and Hong Kong's anti-corruption culture is sure to be of interest to the mainland in coming years.

"I learned from different sources after the Timothy Tong scandal that Beijing is very concerned, and they don't want to see the downfall of the ICAC," said Mr Char, who is a member of a pro-democracy political party.

"Our country is too big. There are too many people. We need a foundation to bring the country forward, and that foundation is the rule of law, where everyone does things law-abidingly."

Xi Jinping, the new Chinese leader, has initiated a high-profile anti-corruption campaign since he came to power.

Stephen Char and Prof Tsao King-kwun both believe Hong Kong's anti-corruption experience can help China as it works to tackle entrenched problems.

There will be a series of special reports and articles this week as the BBC examines why bribes and backhanders are part of the system in so much of the world, looks at countries which have tried to roll back the tide - and explains how corruption works.