"In the immediate years after 1997, nobody wanted [Chinese nationality] … Now everyone who doesn't have it wants it," says immigration consultant Richard Aziz Butt.

The wealth of mainland business interests is a major reason for the appeal of Chinese nationality, Butt says, along with general confidence in the political outlook of the region. "If you want to do business, easy access is essential, and people are gradually realising the 'one country, two systems' is very stable."

The convenience of a HKSAR passport, which offers visa-free or visa-on-arrival access to 150 countries, is also a factor, Butt says.

Annual applications for naturalisation more than doubled from 702 in 2003 to 1,342 in 2004 and have not dipped below 1,200 since then, peaking at 1,840 in 2006.

Veteran banker Marshall Nicholson is a more recent addition, having given up his US passport last year.

"I am now an ABC, American-born Chinese," he quips. "I am happy to be born in America and I am happy to be a Hong Kong citizen."

Managing director at mainland investment bank China International Capital Corporation (CICC), Nicholson moved to the city 12 years ago and has married a Hongkonger.

His motives for seeking naturalisation were first and foremost "family reasons and emotional reasons", he says, followed by business considerations. Moreover, "I travel around so much, I don't know where my home is."

Relinquishing citizenship isn't something people do lightly, and those who do are sometimes attacked as being unpatriotic (Singaporeans who do are labelled quitters, for example).

"One year on," Nicholson says, "I feel comfortable with my decision and people's reaction has been overwhelmingly positive and supportive."

Only 1 per cent of people were "sarcastic around tax issues", he adds, while other critics were "people who are not happy with China and they say 'I can't believe you did that'."

Nicholson now travels to and from the mainland with a home return permit without trouble.

"All the time I travelled to China I have only been stopped twice at the border - and that is when the security guard thought I was standing in the wrong line. When they see my [home return] card, they smile and give me a thumbs up," he says.

"It is a personal decision, it may not be for everybody. For me [the decision] is only about whether I am going to stay here. I have a strong relationship to Hong Kong from family and business point of view."

But while Nicholson's application went through smoothly thanks to an "extremely clean record and good business and personal reputation", the paper chase has become increasingly difficult, especially for ethnic minority residents.

In a quirk of bureaucracy, this quest has unexpectedly turned Sudanese trader Faisal Abbasher into a stateless person.

The 61-year-old has long considered Hong Kong his home, having lived here for nearly three decades. He first arrived in 1982, later married a Filipino woman and gained permanent residency shortly after 1997.

He runs three companies selling electronics, tyres and auto spare parts to Africa, and would frequently visit the mainland to meet suppliers. In fact, he was so busy developing the business he had not seen his family in Sudan - including four sisters and one brother - since 1990.

In 2008, he applied to be naturalised as a Chinese national but was rejected the following year. Meantime, however, the paper work to renounce his Sudanese citizenship had gone through.

As a result, he became stateless, even though he held a Hong Kong permanent ID.

"This is unfair, I feel paralysed," says Abbasher, who speaks conversational Cantonese. "I cannot go to China, I cannot go back to Sudan, I cannot go anywhere in the world except Macau.

"I have no feeling for any state other than Hong Kong."

He has lost business in the past five years, he says, because he hasn't been able to travel to meet customers and suppliers, and often has to pay deposits of up to US$40,000 to place orders without having inspected the products.

No reason was given for the rejection, so Abbasher doesn't know what to do; but he has a sneaking suspicion about why his application failed.

"I feel if I had not been black, the decision would have been different," he says.

Abbasher divorced his wife two years ago and now lives alone. Their 27-year-old son is married with two children, but "they live a separate life from mine".

That's not to say Abbasher feels isolated. "I feel at home in Hong Kong and have many friends here, but it makes me a little unhappy when they go on holiday in Japan and everywhere because only I can't go."

As is its usual practice, the Immigration Department has declined to comment on a specific case.

It also says it had no information on the success rate of naturalisation applications, whether overall or by race.

According to the latest census in 2011, more than 6 per cent of the population is non-Chinese. About half the number are foreign domestic helpers, who were denied eligibility for permanent residency following a landmark ruling by the Court of Final Appeal last year. That leaves more than 200,000 non-Chinese residents who are potentially eligible to be naturalised.

Most applications come from Pakistani, Indonesian and Indian nationals, and informal data suggests they have a success rate above 65 per cent.

However, Immigration Department gatekeepers are getting tougher on who they let through, says Butt, who specialises in naturalisation for ethnic minorities. "They never give reasons but traffic offence history, reliance on CSSA [welfare], and having a criminal record can all mess up your application."

Butt, who was born in Pakistan, was naturalised in 2007 and frequently travels to the mainland on his home return permit.

"I'm Chinese; China is my home," he says with a grin.

Yip Ho-ling, acting executive director (advocacy) of Hong Kong Unison, says the group has received reports from ethnic minority residents that immigration officers have discouraged them from applying, suggesting that their chances were poor. Without statistics from the government, it is difficult to gauge the problem, but discrimination cannot be ruled out, she says.

"The transparency of the process is very low; if the application fails, you are not told the reasons so you don't know whether it is something you can change."

It is disappointing, Yip adds, that it seems as if the government would rather not know whether there was a problem.

For Abbasher, the impact of rejection was particularly painful when his mother passed away in Sudan last month.

As the eldest son in his family, it is the tradition in Sudan for him to give her a burial. But being stateless means "I cannot go to bury my own mother", he says.

"I wish I could live like a normal person; this is like prison."