Messaging app Signal became unusable for many people in mainland China this week, stifling one of the last widely used messaging apps that could send and receive encrypted messages in the country without a virtual private network.

The government’s apparent move to block Signal intensifies its hold on public and private discourse in China, where many social-media and messaging apps, including Facebook, Twitter and, most recently, the popular social-audio platform Clubhouse, have been banned.

Signal users in mainland China started reporting around Monday evening problems with sending and receiving messages in the app. Using a virtual private network, or VPN, a tool that enables internet users to circumvent China’s elaborate system of web filters, resolved those issues, which led users to conclude that the app had been blocked in China.

Some also reported problems with registration, another common censorship practice that affected Clubhouse last month, where users aren’t able to sign up with their phone numbers because the verification text code is never received.

The problems began on Sunday and included halted registration and network blocks, a person familiar with the matter said. The Cyberspace Administration of China, the country’s internet regulator, didn’t immediately respond to a request for comment.

On Tuesday, China’s Ministry of Foreign Affairs referred a question about the apparent ban to the relevant department, but said that China’s internet is open and the government manages the internet in accordance with the law.

Signal uses end-to-end encryption, which prevents third-party access to communications between the sender and receiver. It includes features such as disappearing messages and media, and has been promoted as a tool for secure and private communication. Similar encrypted messaging apps, such as Telegram and Facebook Inc.’s WhatsApp, are also inaccessible in China without a VPN.

Signal surged in popularity last year among Chinese users after the U.S. administration said it would ban WeChat, China’s most popular messaging app, operated by Tencent Holdings Ltd. Downloads also jumped in Hong Kong after lawmakers passed national-security legislation, suppressing pro-democracy protests.

WeChat uses client-to-server encryption, which grants Tencent full access to data between senders and recipients. The app is ubiquitous in China and largely a necessity for everyday life through its messaging and payment services. It is also known for its censorship of sensitive matters, such as of political criticism or during the coronavirus outbreak in Wuhan.

In February, Chinese censors blocked Clubhouse after the app started gaining traction in the country and led to discussions of sensitive topics generally restricted in China, such as the treatment of China’s Uyghur Muslims or the Tiananmen Square protests.

As with Clubhouse, some saw the loss of Signal as inevitable because of China’s expansive censorship apparatus and tightening controls over its internet users.

“It’s always been a surprise that Signal lasted as long as it did, given that the purpose of the app is to facilitate encrypted communications,” said James Griffiths, author of “The Great Firewall of China: How to Build and Control an Alternative Version of the internet.”

Yaqiu Wang, China researcher with Human Rights Watch in New York, said Signal was the last encrypted messaging app that she could easily use to connect securely with friends and activists in mainland China.

Though Signal can still function with a VPN, those have become more difficult to access in China in recent years after China tightened rules over the tool. At least hundreds of VPNs have been removed from app stores, while VPN users and providers have faced fines.

“All of these are indications that it will be harder and harder for people to speak to people securely on sensitive issues," Ms. Wang said. “I really worry about communication for people inside China.”

While there was no clear catalyst for the apparent ban, some experts speculated that Signal’s recent popularity with mainland users may have contributed.

According to research firm Sensor Tower, downloads of Signal on iOS, Apple’s operating system, had been gaining momentum in China over the past year before dropping off in February and March. Signal installs reached a monthly high in August of 52,000 after the U.S. WeChat ban was announced and surged again in January, with about 49,000 installs.

Still, Signal users in China are a fraction of those on WhatsApp or Telegram, according to Sensor Tower data. Signal downloads on iOS have reached a total of about 510,000 in China, compared with 9.6 million WhatsApp installs and three million Telegram installs.

Signal was also blocked in Iran in January. The company has said it launched a workaround for that network block and was exploring more ways to circumvent the ban.