Four more women have spoken of suffering abuse during relationships with Islamic scholar Tariq Ramadan, who is now facing an online petition calling for his suspension from his prestigious teaching post at Oxford University.

It is understood the four women have not filed formal complaints, but have been in touch with French journalists to describe their experiences.

The latest development follows a string of damning rape and sexual assault allegations about Mr Ramadan, a professor at Oxford University and the grandson of the founder of the Muslim Brotherhood in Egypt.

Mr Ramadan has denied the allegations and is counter-suing for libel.

Caroline Fourest, a French author who has written a book and a documentary on Mr Ramadan, told The National that she began hearing complaints about him as far back as 2009. The four latest women who have made direct claims about Mr Ramadan's behaviour all live in Europe – in France, Belgium and Switzerland.

The consensual relationships with Mr Ramadan are alleged to have turned violent or abusive. "Not all were raped — they didn't all experience the same amount of abuse," Ms Fourest said. "I met two girls who were trapped in a sadistic relationship, and one who was a victim of rape with hard violence. Another was vague about what she experienced."

Accusations about Mr Ramadan's love life have circulated in European newspapers since late last month. One woman, Majda Laroussi, published pictures of Mr Ramadan half naked in a hotel room. Another, Lucia Canovi, spoke to other French journalists and says she is in contact with many more victims. Sexually explicit screenshots of Mr Ramadan on a Skype call, taken by an unnamed woman, have also been leaked to The National.

"What they describe confirms he is a Machiavellian character," Ms Fourest said. "A predator is very smart to take advantage of girls who were vulnerable and target those who will be too scared to complain. In that case, the girls are too ashamed to speak up, and frightened by the Muslim Brotherhood. He had double standards, in his personal life as well as his professional life."

She added: "My real fear is that we are only hearing from women in Europe. Mr Ramadan has been on countless trips around the world, giving lectures and speaking at conferences over the past 30 years. What about the women he encountered in Senegal, Yemen and Qatar? Will they be able to make complaints?"

Those who have made complaints say they have been inundated with insults and abuse.

French writer Henda Ayari, who lodged a rape complaint against the 55-year-old Swiss national on October 20, told The New York Times that she was subjected to an onslaught of vitriol since naming him as her attacker.

"The reaction, the buzz, really frightened me," she said. "I am very scared of being recognised when I go out in the street. I am scared that they will hurt my children, that they know where I live. It is very hard."

Ms Ayari has also received online death threats on her Facebook page.

"If complainants in France are receiving death threats, you can imagine how hard it is for women in other countries, in the Middle East and North Africa, to speak up," Ms Fourest said.

Mr Ramadan has said the allegations are part of a campaign by his enemies to blacken his name. In a Facebook post last week, he said a new legal suit would follow "within a few days, in response to the campaign of lies launched by my adversaries". He did not respond to requests for comment.

However, Ms Fourest has no doubt that the complaints are genuine. "Maybe some of the women were seduced by him, but they didn't give their consent for rape and violence," she said.

Oxford University is already facing anger at its decision to allow Mr Ramadan to continue teaching despite the mounting allegations of rape and sexual assault. Mr Ramadan is professor of Contemporary Islamic Studies at St Antony's College in Oxford, which is financed by Qatar.

Aisha Ali-Khan, a prominent women's activist in the UK, has filed a petition asking Oxford to suspend Mr Ramadan while the investigations are underway.

Ms Ali-Khan told The National that she had heard reports of Mr Ramadan's sexual misconduct going as far back as 2008. In 2011, she spoke to an officer of the Metropolitan Police, the London police force, about a possible victim in the UK but no charges were filed and the matter was later dropped.

On Friday, Ms Ali-Khan submitted a Freedom of Information request to the Met Police, demanding to know if any other women have lodged complaints against Mr Ramadan in the past. She has yet to hear back.

"Following the most recent allegations as revealed by The Tribune de Genève and The National, questions are now being raised about the safety of the female students who encountered Mr Ramadan in his role at Oxford," Ms Ali-Khan said.

"Parents whose daughters were in Oxford are now bound to ask: 'Could our daughters have been subjected to sexual harassment by him given the nature of the allegations that are now coming out?'"

She added: "I am really afraid of the numbers could be incredibly high, especially if some of his victims are Muslim women who may be reluctant to come out and speak about sexual assault in case it compromises their or their family's honour.

"This could just be the tip of the iceberg."