Further doubts about Jeremy Corbyn's credibility as a potential Labour leader were raised after he was forced into an embarrassing u-turn over his links to a former Hizbollah fighter.

Mr Corbyn said in 2009 that he would lead a campaign against the Home Office's decision to ban Dyab Abou Jahjah from entering Britain.

In an interview on BBC Radio 4's World at One on Wednesday, the MP denied any knowledge of Jahjah. Asked about the activist, Mr Corbyn responded: "Sorry, who? I saw the name this morning and I asked somebody: 'Who is he?'" Pushed on his links to Jahjah, he added: "I'm sorry, I don't know who this person is."

A picture of the two men sitting next to each other at an event in Parliament in 2009 had appeared in two newspapers only hours earlier. That forced Mr Corbyn into reversing his position later in the day. "My staff have researched this and told me that I did meet this man," he admitted.

Jahjah, the leader of a radical Muslim group, had earlier boasted of twice having met the man likely to become Leader of the Opposition. In a blog post Jahjah denied being antisemitic and said he had "met and collaborated" with Mr Corbyn in 2009.

The Lebanon-born activist added that he was "opposed to Zionism and to the policies of the state of Israel. I am indeed a supporter of a one-secular-state-for-all solution in Palestine in the line of the South African dismantling of the apartheid state."

Jahjah was jailed in Belgium for fomenting violence in 2002 when Muslims rioted after the murder of a Moroccan teacher. In 2007, he caused outrage when he claimed to have published a picture of Anne Frank in bed with Adolf Hitler alongside the caption "put this in your diary, Anne".

He has also reportedly questioned the existence of gas chambers during the Holocaust, said he considers the death of every Dutch, British and American soldier a "victory", and claimed gay people are "Aids-spreading faggots".

In reference to his comments about the deaths of British soldiers, Jahjah wrote on Wednesday: "Every soldier taking part in an illegal occupation is a legitimate target for resistance."

When the JC reported Jahjah's ban from Britain in 2009, he blamed the newspaper for prompting immigration officials to detain him for six hours.

He wrote at the time: "It is funny they only discovered that after my successful visit… and after granting me access a few days ago. I think it all has mainly to do with the lobbying of the Zionists who again have a hate article full of lies on me in today's [April 3] issue of the Jewish Chronicle."

The then Board of Deputies president Henry Grunwald had written to the Home Secretary of the time, Jacqui Smith, to raise concerns about Jahjah's presence in Britain.