The far-right campaigner Tommy Robinson is being threatened with libel action by lawyers representing a Syrian refugee who was allegedly attacked at school.

Robinson, who posted a series of videos and Facebook posts about the incident in October, has had a letter seeking to sue him for defamation delivered to his family’s home. Members of the public donated £10,300 to help fund the legal action against the English Defence League founder.

Footage of the 15-year-old victim, who can be identified only as Jamal, being pushed to the ground at his Huddersfield school and having water poured on his face attracted widespread condemnation, including from Theresa May, in December.

A 16-year-old boy who cannot be identified was charged with assault and appeared before a youth court last month.

Hours after the video went viral, Robinson claimed on Facebook that Jamal had previously attacked three schoolgirls and a boy.

The anti-Islam activist, whose real name in Stephen Yaxley-Lennon, reposted a screenshot of a message on his Facebook page from a mother claiming her daughter had been bullied. However, the mother later posted on Robinson’s page denying that it was Jamal who had allegedly attacked her daughter.

Robinson posted a series of videos on his Facebook account accusing Jamal of bullying and claiming “lots of Muslim gangs are beating up white English kids” in Britain. Robinson’s Facebook page had more than 1 million followers and the posts on the Huddersfield incident were viewed up to 900,000 times each.

Tasnime Akunjee, a solicitor who represents Jamal and his family, alleges that the posts were defamatory and that Facebook allowed them to spread across the world.

Akunjee said: “What has gone out to him is a ‘letter before action’; it is a document which sets out Jamal’s claim against him for defamation and breach of GDPR [data protection] rules.

“Mr Lennon has 14 days to respond either with an offer to settle that is acceptable to Jamal or with notification that he intends to defend this claim. If he fails to respond within the timeframe, we are instructed to issue proceedings against him in court.

“Our submission is that Mr Robinson, in tandem with Facebook, engaged in defamation of our client and these the claims were repeated numerous times across the world.”

On a page raising money for the legal action, Abdulnaser Youssef, of Farooq Bajwa and Co solicitors, wrote that allegations made against Jamal led to threats that forced him and his family to move.

He wrote: “As a result of Lennon’s false remarks and their wide online circulation, Jamal became the focus of countless messages of hate and threats from the extreme rightwing. The police informed Jamal that there was an increased risk to the safety of he and his family; as a result, he and his family were forced to relocate.”

Facebook deleted several of Robinson’s videos for violating community standards after the Jamal’s family announced their intention to sue in November.

On Tuesday the page was removed as Robinson was permanently banned from Facebook and Instagram for repeatedly breaking policies on hate speech. Facebook said he broke rules that ban public calls for violence against people based on protected characteristics; rules that ban supporting or appearing with organised hate groups; and policies that prevent people from using the site to bully others.

Robinson said by email that the delivery of the letter entailed “intimidating an innocent woman and her children by sending 5 men with a dog to the house whilst I wasn’t even in the country.”

Akunjee said that the police were present when the letter was delivered.