Apple censors references to Chinese politicians, dissidents and other topics in its engraving service, a report alleges.
Citizen Lab said it had investigated filters set up for customers who wanted something engraved on a new iPhone, iPad or other Apple device.
And Apple had a broad list of censored words, not just in mainland China but also in Hong Kong and Taiwan.
Apple said its systems "ensure local laws and customs are respected".
"As with everything at Apple, the process for engraving is led by our values," chief privacy officer Jane Horvath wrote in a letter provided to CitizenLab in advance of the publication of its report.
And the engraving service tried not to allow trademarked phrases, alongside those that "are vulgar or culturally insensitive, could be construed as inciting violence, or would be considered illegal according to local laws, rules, and regulations".
But CitizenLab accuses Apple of having "thoughtlessly and inconsistently curated keyword lists".
CitizenLab, a research group at the University of Toronto known for its work in technology and human rights, said there had been previous research on the censorship of Apple's App Store in China.
But there were until now only anecdotal reports of engravings being refused, it said.
Its new report found more than 1,100 filtered keywords, across six different regions, mainly relating to offensive content, such as racist or sexual words.
But it alleges the rules are applied inconsistently and are much wider for China.
"Within mainland China, we found that Apple censors political content, including broad references to Chinese leadership and China's political system, names of dissidents and independent news organisations, and general terms relating to religions, democracy, and human rights," it says.
The report also alleges that censorship "bleeds" into both the Hong Kong and Taiwan markets.
- 1,045 keywords blocked in mainland China
- 542 in Hong Kong
- 397 in Taiwan
In contrast, Japan, Canada and the US had between 170 and 260 filtered words.
In Hong Kong, phrases referencing the "umbrella revolution", pro-democracy movement, and freedom of the press appeared to be blocked, along with the names of some political dissidents.
In Taiwan, the report found filtering of senior members of China's ruling Communist Party, including historical figures such as Chairman Mao Zedong.
Hong Kong is what is known as a special administrative region of China.
The former British colony is part of China but governed under special principles and enjoys a high degree of autonomy.
Taiwan, meanwhile, is self-governing but Beijing considers it a breakaway rebel province that will one day be reunited with mainland China.
"Much of this censorship exceeds Apple's legal obligations in Hong Kong and we are aware of no legal justification for the political censorship of content in Taiwan," the report says.
It also cites mistakes - such as 10 people with the surname Zhang having their engravings censored, a restriction with no obvious political significance.
"Apple does not fully understand what content they censor," CitizenLab alleged.
"Rather than each censored keyword being born of careful consideration, many seem to have been thoughtlessly reappropriated from other sources," it said - possibly including a list used to censor products at a Chinese company.
China was a valuable market for big technology companies, CitizenLab said.
But its research "points to a more alarming trend of the export of one jurisdiction's regulatory and political pressures to another".
There were "growing uncertainties and dilemmas global companies face between upholding internationally acknowledged human-rights norms and making decisions purely based on commercial interests", it added.
Replying to the group, Ms Horvath said Apple's rules depended on the region - and "no third parties or government agencies have been involved in the process".
"To a large degree, this is not an automated process and relies on manual curation," she said.
"At times, that can result in engraving requests being mistakenly rejected.
"And we have a process in place to review and correct those situations when they occur."