Cyber-activists have retaliated against Chinese authorities' censorship of foreign media websites by exposing an apparent weakness in the country's vast internet control apparatus.

China blocked the Wall Street Journal and Reuters Chinese-language websites on Friday after a New York Times exposé revealed business ties between JP Morgan and the daughter of the former premier Wen Jiabao. Both websites appear to still be blocked on Monday. The New York Times's English and Chinese-language websites have been blocked in China since 2012.

Charlie Smith, the co-founder of GreatFire.org, a website which monitors internet censorship in China, says he has helped discover a strategy to make these sites available in mainland China without the aid of firewall-circumventing software.

"We think we have exposed a weakness in the Great Firewall," he said. The strategy involves creating mirror websites – essentially replicas of existing sites – which authorities would be unable to block without severely disrupting other, government-sanctioned internet traffic. A mirror site that GreatFire.org established for Reuters' Chinese-language site is currently accessible within mainland China.

"We're serving these mirror sites through companies like Amazon," Smith said. "For them to block these mirror sites, they're going to have to take down Amazon web servers in China, and that would affect thousands of services in China, maybe tens of thousands," he said. Many Chinese websites hosted by Amazon's web services are involved in e-commerce, he said, so a blanket ban could have significant economic consequences.

The Chinese government has long used a range of intimidation tactics, including internet censorship, visa denials and verbal warnings, to express its displeasure with news agencies that it deems a political threat. Authorities blocked the websites of Bloomberg News and the New York Times in 2012 after they featured lengthy investigations exposing the wealth amassed by family members of top leaders. They have withheld visas for incoming reporters from both organisations, some for more than a year.

The blocks come amid a controversy over self-censorship, as western media companies vie for financial gain in the world's second-largest economy. Top editors at Bloomberg News allegedly quashed two politically sensitive investigations last month to avoid jeopardising the organisation's China bureaux, the New York Times reported. Most of Bloomberg's revenue in China comes from subscriptions to financial terminals, and sales have slumped following publication of sensitive articles in the past.

Bloomberg managers have suspended the Hong-Kong-based correspondent Michael Forsythe – a lead reporter on one of the stories – for leaking the editorial decision, the New York Post reported on Friday. "Thanks everyone for the incredible outpouring of sympathy and support," Forsythe tweeted on Monday evening in his first public statement since the suspension. "It has really helped me and my family get through this.

The Reuters Chinese-language website was blocked soon after it published news concerning the New York Times exposé, Smith said. The Wall Street Journal site was blocked at around the same time, although the reasons appear less explicit. "All of this stuff is related to the news about Bloomberg – which media organisations are self-censoring at the moment, and which aren't?" he said. "Financial Times Chinese isn't blocked. What does that mean?"

Bloomberg has vociferously rejected allegations of self-censorship. "It is absolutely false that we postponed these stories due to external pressure," a spokesperson, Belina Tan, said in an email. "We are disappointed that they chose to publish a piece that claims otherwise."

Early this month, China's foreign ministry notified Paul Mooney, a 63-year-old reporter with 18 years of experience in China, that his visa application to work for Thompson Reuters in Beijing had been denied. He had been waiting for eight months.

Judging by a "tough" visa interview he had undergone at San Francisco's Chinese consulate this spring, he said, authorities were probably displeased with his reporting on sensitive political issues such as detained human rights lawyers and ethnic tensions in Tibet. "The consular officer said: 'If we give you the visa and allow you to go back, we suggest you be more objective in your reporting,'" Mooney said in a phone interview. "It's a form of intimidation."

Mooney returned to the US in 2012 when the visa sponsored by his former employer, the South China Morning Post, expired. "I always considered myself a China person first and a journalist second," he said. "So it's hard to think that I won't be able to go back and continue the work I was doing."

• This article was amended on 20 November 2013. An earlier version said GreatFire had established a mirror site for the Wall Street Journal that was accessible within mainland China. That is not the case.