An investigation into allegations that Sheffield Hallam University faced sustained pressure from China to shut down human rights research has been referred to counter-terrorism police.

The BBC and the Guardian newspaper has been reporting that documents show that China had waged a two-year campaign of intimidation and harassment, including demands the university stop sensitive research by one of its professors into claims of forced labour in the Xinjiang region of China.

A South Yorkshire Police spokesperson said the force has referred the investigation on because the “allegations fall under Section 3 of the National Security Act”.

Section 3 of the act deals with “assisting a foreign intelligence service”.

An offence is committed if someone behaves in a way that “intends that conduct to materially assist a foreign intelligence service in carrying out UK-related activities,” or in conduct likely to assist that service.

Internal documents from Sheffield Hallam University were released under a Subject Access Request to Professor Laura Murphy, whose research is alleged to have been targeted.

They showed university staff in China had been threatened by individuals identified as being from China’s National Security Service, who demanded the research by Prof Murphy taking place in Sheffield be shut down.

The documents went on to state that “a decision by the university not to publish a final phase of the research on forced labour in China was communicated to the National Security Service… Immediately relations improved and the threat to staff wellbeing appears to be removed”.

China is not believed to have an organisation named National Security Service, so it is not clear who the individuals were.

The decision referred to in the documents is alleged to have led to Prof Murphy being told by the university that she could not continue her work on China at the start of this year – then reversing that position after she started legal action for infringement of her academic freedom.

In a statement issued earlier Sheffield Hallam University had apologised to Prof Murphy.

A spokesperson said: “The University’s decision to not continue with Professor Laura Murphy’s research was taken based on our understanding of a complex set of circumstances at the time, including being unable to secure the necessary professional indemnity insurance.”

The insurance for social sciences staff had been suspended by the university’s insurers as a Chinese firm had brought a defamation suit against Sheffield Hallam after it was named in a report she produced. That lawsuit is ongoing.

The university spokesperson added: “Following a review, we have since approved Professor Murphy’s latest research and are committed to supporting her to undertake and disseminate this important work.

“For the avoidance of doubt, the decision was not based on commercial interests in China.

“Regardless, China is not a significant international student market for the University.

“We have apologised to Professor Murphy and wish to make clear our commitment to supporting her research and to securing and promoting freedom of speech and academic freedom within the law.”