HONG KONG — Software made by foreign companies to help Chinese users skirt the country’s system of internet filters has vanished from Apple’s app store on the mainland.

One company, ExpressVPN, posted a letter it received from Apple saying that its app had been taken down “because it includes content that is illegal in China.”

Another posted a message on its official account that its app had been removed.

A search on Saturday showed that some of the most popular foreign virtual-private networks, also known as VPNs, which give users access to the unfiltered internet in China, were no longer accessible on Apple’s app store there.

ExpressVPN wrote on its blog that the removal was “surprising and unfortunate.”

It added, “We’re disappointed in this development, as it represents the most drastic measure the Chinese government has taken to block the use of VPNs to date, and we are troubled to see Apple aiding China’s censorship efforts.”