Slide open the drawer beside your hotel bed and, in many cities, you will find a Gideon's bible. In Macau you discover a small, neat, plastic-wrapped totem of another faith: a brand new deck of cards. Even when you have closed your curtains to the blazing neon, the casinos are with you in spirit. Each year, millions of visitors, rich and poor, make the pilgrimage from Hong Kong and the Chinese mainland. The wealthy elite are helicoptered in. Factory workers arrive on cramped coaches. Their temples have names such as the City of Dreams and Babylon; the congregations are growing.

They used to call it Asia's Monte Carlo, or the Las Vegas of the East. But this small territory – barely known to many in the west – is the new giant of the casino industry. For centuries, Macau was a Portuguese colony; since 1999 it has belonged to China as a Special Administrative Region. Thanks to the relative freedoms it enjoys under this "one country, two systems" formula, it has leapfrogged its rivals, embracing the glitz and kitsch of gambling; and, above all, the cash.

These days Macau is bigger than Vegas: four times bigger, to be precise. Last year, gaming revenues from its 33 casinos hit a record high of £14.7bn. In April they surged to a new monthly record of £1.56bn – well above the entire annual takings for 2001. Accountancy firm PricewaterhouseCoopers estimates that the yearly take could hit £27.5bn by 2014.

Yet some of the shine has come off the industry recently. The government has vowed to rein in casino development; and US and Hong Kong regulators have launched inquiries into one of the big operators. Could the sector's long streak of luck be running out at last?

The overnight success of Macau's casino industry was more than a century and a half in the making. In the 1840s, Portuguese administrators searched for new sources of revenue as foreign merchants and traders decamped to fast-developing Hong Kong.

"They effectively took up all the marginal and grotty jobs and trades," explains historian Jason Wordie: selling indentured labourers to Latin American mines and plantations; licensing brothels; and regulating gaming houses. Camilo Pessanha, a Portuguese poet who lived in Macau for many years, described it as "a material and moral rubbish heap".

But for the residents of Hong Kong and southern China, the appeal was simple. "It was close enough to be accessible – and far enough away from prying eyes," says Wordie, whose book Macau: an Exploration is out next year. "It was the same as today: for people keeping mistresses or gambling."

Gaming tourism got a kickstart in the 1920s from an energetic new monopoly; and another boost in the 60s, when the government granted the rights to a syndicate dominated by Stanley Ho. Until March, the 89-year-old "casino king" held around a third of the industry. Now, after a very public feud that saw him threatening to sue two of his 17 children (by four wives), he has announced he is handing over most of his stake to his family. His empire includes perhaps Macau's best-known casino, the Lisboa, which these days has the seedy air of a down-at-heel Blackpool nightclub. Its neon displays have a retro feel and saxophone jazz eases from the speakers. "No singlets, no slippers, no shorts" reads a sign at the entrance. Inside the ceilings are low, the lighting dingy and the air thick with stale smoke.

It is dwarfed by its offspring, the Grand Lisboa, just across the road. Dominating the skyline for miles, the edifice is a monument to vulgarity and Macau's new prosperity. Though reportedly inspired by the region's lotus symbol, it resembles nothing so much as an enormous, gilded onion, with a flashing globe studded with 1.2m LEDs and long glass shoots ascending into the sky. In the lobby, tourists marvel at the artwork: giant fantasias of peaks and temples; gold peacocks with spreading enamel tails. On the gambling floors, rows of Bank Buster and Black Gold Wild slot machines flash wildly and even the baize looks brighter. The lights in the ladies' toilets are crystal extravaganzas.

Macau has the Chinese Communist party to thank for this transformation. The late 90s were a nadir for the region, with powerful interests jockeying for position as the gambling concession drew to a close and Portugal prepared to hand the colony to the mainland. Turf wars spiralled out of control; 1999 saw 42 murders in a territory with just half a million inhabitants. "There were bombs going off, assassinations and out-and-out carnage," says Stephen Vickers of International Risk, formerly a senior Hong Kong police officer who spent years taking on triads. "Broken Tooth [a triad leader] saw an opportunity to move in and force his way through. He was ruthless."

Then Beijing took charge, marching the military into the centre of Macau and warning the gangs that the violence had to end. Broken Tooth and his lieutenants were jailed for criminal association, loan-sharking and illegal gambling. More importantly, the handover unleashed the mainland's pent-up demand for gambling. Its casinos have flourished not in spite of the communists' longstanding ban on mainland gaming – but because of it.

"Chinese people love gambling, so they love to come to Macau," said a Fujian entrepreneur, too coy to give his name as he took a cigarette break by the Wynn casino's Performance Lake. "There's a lot of 'black' [illegal] gambling on the mainland. But it's safer to play here and people come to have fun. You don't get things like this at home."

In fairness, there are not many places in the world to watch water jets spurt and fireballs erupt in time to a pounding cover of Holding Out For a Hero. Such spectaculars are testament to the two factors that turbo-charged Macau's expansion. The first was the decision to liberalise the industry, introducing a massive influx of foreign capital and Vegas razzmatazz from American gaming giants Wynn, MGM and Sheldon Adelson's Las Vegas Sands. The second was China's astounding economic growth, producing a stream of visitors who could never have afforded the trip before. In 2001 there were just 3m trips to Macau. Last year there were 25m, mostly from the mainland.

On peak days during the Chinese new year as many as 120,000 tourists pour through the doors of the Venetian Macau. Fortunately, it is the world's largest casino, with 550,000 square feet of gaming space. It is the ultimate in ersatz tourism – a replica of a replica – with all the gilding and gondolas from Nevada and twice the space. Tourists pose for photos in "St Mark's Square" and soak up the Venetian-style entertainment: living sculptures and American gondoliers with fake Italian names and faker accents.

Despite such exotic features, Macau's casinos are carefully tailored to the Chinese market. Feng shui consultants have determined the layout; players snack on congee or dumplings; the complimentary drink is tea, not beer. There's little poker and less roulette, but endless baccarat: the game accounted for more than 85% of revenues in 2008. The biggest difference is the players' demeanour; the intensity with which they peel cards from the table and throw them back towards the dealer. The clack of chips and shuffle of decks is punctuated not by chatter or laughter but by the occasional, angry thump of a table. Gaming is a serious business.

This helps to explain the paradox that confounds many observers: that a culture with one of the highest saving rates in the world is so drawn to gambling. In part, the habit is self-sustaining. Chinese people have been gaming for millennia; "You have a general culture where people will bet on two flies crawling up a wall," says Wordie.

But gaming expert Dr Wang Xuehong, of Peking University, suggests there is more to it. Everyone enjoys taking risks – "like being married", she explains – but westerners see betting as a leisure activity, while Chinese players regard it as a financial venture. "They are taking a risk, but it's taking a risk so that they can make more money." Studies also show that Chinese gamers have a high illusion of control, believing they can influence the outcome of what are largely games of chance. "People are superstitious and believe they have good luck. It's not really an investment – but it looks like an investment to them," she says.

You can watch average-looking customers drop hundreds of pounds in just a hand or two. But the real action takes place well away from the casino floors, in the VIP rooms; around three quarters of Macau's gaming revenues comes from the thousands of high-rollers. Although the casinos offer perks, from Jacuzzi-furnished suites to flights in private jets, they depend on junket operators – a by-product of Beijing's currency controls – to bring in these wealthy players. Serious gamblers deposit their yuan on the mainland and the junket firms advance them money in the casinos' VIP rooms – often extending substantial credit.

"My experience is that western-owned casinos do their best to prevent wholesale bad activity inside their premises. But junkets are notoriously difficult to control," says Vickers. "My personal assessment is that many are triad-controlled. It may be that the registered operators are clean – but the hard reality is that financiers or guarantors behind them primarily have triad backgrounds of some sort." He suggests the gangs have "gentrified" in the past decade: "On the surface the street crime has diminished. [But] the pie is much larger – there is money laundering, loan sharking, prostitution and all the other fun stuff."

There have been repeated scandals over Chinese officials using trips to Macau to gamble away embezzled public money or launder bribes, one reason authorities have mixed feelings about the industry. In 2008 the gambling trade wobbled when Beijing tightened travel restrictions to the region, hitting casinos already reeling from the financial crisis. The sector faced another challenge in March when shares in Sands China, which owns the Venetian and Sands Macau, fell on its announcement that Hong Kong regulators were investigating alleged breaches of financial regulations.

Earlier, its parent company Las Vegas Sands said the US Securities and Exchange Commission and justice department were investigating it in relation to the Foreign Corrupt Practices Act. Executives have linked those inquiries to the wrongful termination case brought by the former chief executive Steven Jacobs. The firm, which is contesting the suit, said it sacked him for exceeding his authority; Jacobs has alleged he was fired after resisting "outrageous" demands to use "improper 'leverage'" against Macau officials.

"Neither the SEC nor the Department of Justice has accused the company of any wrongdoing. The subpoena is described as a fact-finding inquiry and does not mean the SEC has concluded anyone has broken the law," says a Sands spokesman. Sheldon Adelson, the tycoon behind the group, pledged in March: "We're going to be found absolutely clean."

Others have broader concerns about the impact of casinos. It does not seem hard to avoid the industry; in parts of Macau you could be in a low-key Hong Kong neighbourhood, or even – thanks to its colonial architecture – a Mediterranean fishing village. But critics point to largely unremarked effects. The Yat On Pathological Gamblers Counselling Centre reports a rise in problem gaming, in many cases among casino employees. "When society is promoting gambling as its pillar industry, it must fulfil its social responsibilities as well," warns director Mee Kam.

Employment has undoubtedly increased in Macau – almost 15% of the labour force are in gaming or gaming-related work – and per capita GDP soared to £30,500 last year; one of the highest rates in the world. Yet half of workers earned less than £8,280. Inequality is soaring and while the rich get richer, rising property and living costs have outstripped the incomes of poorer residents.

Much as Macau officials enjoy the whopping revenues netted by an effective tax rate of 39%, they say they want to diversify development. Yet Aaron Fischer, director of consumer and gaming research for the CLSA brokerage and investment group in Hong Kong, predicts growth of 20 to 30% annually over the next five years. "The government cannot control revenue growth: all they can do is control the supply. They do not want the 60% growth we saw last year; it wants to send the message to Beijing: we will control growth," he says.

Casino operators say they are embracing diversification, expanding into exhibitions and conventions and developing family attractions. The Venetian has hosted Beyoncé concerts and has its own Cirque du Soleil show. Resorts are crammed with designer boutiques and retail sales rose four-fold between 2004 and 2010, to £2.25bn.

"Las Vegas was a hardcore gambling city 50 years ago," points out Dr Desmond Lam of the University of Macau, author of The World of Chinese Gambling. "It takes a while to change, but things are going in the right direction."

Even so, it would take a lot of sales of Gucci bags and tickets for Sarah Brightman concerts to even approach the income from gaming tables. The truth is that Macau – like the visitors who stream here – is placing its faith in the casinos, and praying that its luck holds.