Universities were urged to keep classrooms clear of remarks that "defame the rule of the Communist Party, smear socialism or violate the constitution and laws". Teachers must also not grumble in class to avoid "passing on negative emotions to their students", Yuan reportedly said.

Citing a joint directive from the State Council and the Central Committee's General Office, Yuan said teachers must "stand firm and hold the political, legal and moral bottom line".

Peking University law professor He Weifang - who was named by party journal last week as an example of a university teacher "defaming" the Chinese legal system by spreading Western ideology - said there had always been conservative voices stressing the importance of Marxism in textbooks.

"But it is particularly strong and has even become an official measure this year," he said.

He said Yuan's remarks amounted to administrative interference in academic freedom and ran counter to the "modern trend".

"So many children, including those of senior party officials are studying abroad. Does the Ministry of Education plan to do something about it as well?" He asked.

Earlier this month, universities were told to step up propaganda and teaching of Marxism and Chinese socialism to ensure such values "get into the students' heads". The institutes would be assessed on their use of set textbooks on Marxism.

Yunnan University journalism professor Guo Jianbin said Yuan's remarks signalled a push for greater ideological control of society, but it was impractical to abide by his words to the letter.

"Times have changed and too much Marxism or leftist talk in university classrooms could easily backfire," Guo said.

"Minister Yuan should learn that criticism is the right of being an intellectual and it does not amount to a smear."