Tommy Robinson has been moved to a "closed wing" in prison after reports emerged that he had a "mark on his head" and he would be killed by another inmate, the High Court has heard.
The activist, whose real name is Stephen Yaxley-Lennon, is currently serving an 18-month sentence for contempt of court.
His legal team argued in the High Court on Thursday that he should be allowed to challenge his segregation from other prisoners at HMP Woodhill in Milton Keynes.
The Ministry of Justice is opposing the bid.
Tommy Robinson admitted 10 counts of breaching the orderPA
HMP Woodhill in Milton Keynes
PA
Robinson's barrister, Alisdair Williamson KC, told the court his client had suffered "an evident decline in his mental health".
In February, Robinson requested to be moved to an open prison, stating: "The solitary confinement is destroying my mind."
He added: "I am terrified of the long-term consequences of the continued solitary confinement. I feel I am being provoked to react."
Robinson claimed in a separate complaint that his "continued isolation" was "politically motivated because of my activism and beliefs."
Tommy Robinson was arrested under anti-terror laws, according to his social media account
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Cross told the court that Robinson's segregation was "more permissive" than usual arrangements. He has access to a laptop and can send and receive "thousands of emails."
Robinson also gets three hours out of his cell daily to use the exercise yard and gym facilities. His cell contains a TV, CD player and DVD player.
Prison governor Nicola Marfleet said in a statement that Robinson has "significantly more visit time than any other prisoner" with two-hour visits four times weekly.
He receives daily visits from both chaplaincy staff and medical professionals. Williamson argued that while there was a threat to Robinson at HMP Woodhill that justified the original decision to segregate him, the continued isolation was harmful.
Stephen Yaxley-Lennon, also known as Tommy Robinson, addresses the crowd
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Williamson said: "We are presented with a position that Woodhill is not a safe place for him to be unless he is in segregation, but that segregation is, we suggest, harming him."
The barrister noted that HMP Woodhill has "a significant Muslim population", adding that "it appears to be that which is causing a difficulty".
He argued Robinson should be moved to a different prison "where he does not have to be kept in isolation".
Robinson was jailed in October last year after admitting 10 breaches of a High Court order made in 2021. The order barred him from repeating libellous allegations against a Syrian refugee who successfully sued him.
The case stemmed from a 2018 incident when Robinson made false claims about Jamal Hijazi, a Syrian refugee who was filmed being attacked at school. Robinson is set to be released from HMP Woodhill on July 26.
At the end of Thursday's hearing, Justice Chamberlain said he would aim to give a decision in writing on Friday.