Reuters Gambling is illegal on mainland China, and advertising Macau's casinos to mainlanders is illegal.

Chinese President Xi Jinping has officially declared war on the global gambling industry, warning foreign casinos that Chinese citizens will be gambling much less in China, neighboring countries, and the US.

"Some foreign countries see our nation as an enormous market, and we have investigated a series of cases," said Hua Jingfeng, a deputy bureau chief at the Ministry of Public Security . "A fair number of neighboring countries have casinos, and they have set up offices in China to attract and drum up interest from Chinese citizens to go abroad and gamble. This will also be an area that we will crack down on."

In other words, Xi is telling companies around the world that saw their revenue triple when Macau opened up to foreigners that the Chinese gambler will not be following them abroad to countries like Singapore and the Philippines where billions have been spent on new projects to attract those same people.

Asian companies have heretofore capitalized on the traveling Chinese gambler in the Philippines, Australia, and especially Singapore. A combination of tax credit and increased gambling revenue in Singapore saved Las Vegas Sands' fourth-quarter earnings. Now that's set to take a hit.

And Chinese gamblers apparently won't be going to Macau as much to gamble, either. Xi's government has sworn to cut down on advertisements — sometimes in the form of annoying text messages — promoting Macau casinos. Visa restrictions will tighten, according to analysts, and even retail gamblers will have their money tracked as officials monitor UnionPay, the only domestic bank card in mainland China.

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Reuters Chinese President Xi Jingping.

The latest announcement goes above and beyond the pain Xi's administration has already put Macau's casinos, and the world's casino companies, through over the past year. The president's anticorruption crackdown has slowed the movement of mainlanders to the island — the world's largest gambling center — dramatically.

Gaming revenue in Macau contracted for the first time since foreign companies were allowed to do business on the island in 2002. It plummeted 30% in December alone. The junket system, Macau's financing system for high rollers most affected by the anticorruption drive, is on life support after 16% of junkets closed in 2014.

So Xi's declaration is coming when companies around the world — from MGM to Las Vegas Sands, from Macau-based companies Melco to SJM Holdings — are at their most vulnerable.

View photos Melco Crown Entertainment gaming resort City of Dreams and the first Hard Rock Hotel in Macau More

Reuters Melco Crown Entertainment gaming resort City of Dreams and the first Hard Rock Hotel in Macau.

Casinos, especially foreign firms, have given no indication that they saw this coming. On a conference call earlier this week, Steve Wynn, CEO of Wynn Resorts, lamented his company's weak fourth-quarter earnings, and struck an almost defeated tone about the government's control over the future of his business.

"China remains a big question mark. We have more questions than answers, thousands of our Macau employees are anticipating promotion and a better life because of Wynn Palace ... We have learned in the last 12 years to behave in China, and that is to listen carefully to what the leadership says and to conform with the program as we are their guest," said Wynn. "We wait for an announcement from the government with baited breath ... What we are seeing in China is an entrenchment."

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