Jeremy Corbyn has faced questions about a wreath-laying ceremony he attended in Tunisia in 2014.
The Labour leader has said he visited the cemetery to honour innocent people killed in a 1985 Israeli air strike.
But he has faced criticism because the event took place near memorials for people who were accused of having links to a terrorist attack at the 1972 Olympic Games.
In recent days the issue has returned to the headlines. Here's what we know:
Why was Mr Corbyn in Tunisia?
In September 2014, when still a backbench MP, Mr Corbyn attended a conference in Tunis, organised by the Centre for Strategic Studies for North Africa.
It was called the International Conference on Monitoring the Palestinian Political and Legal Situation in the Light of Israeli Aggression.
One of its aims was to reconcile differences between the two main Palestinian factions: Hamas and Fatah. Other attendees included former US attorney general Ramsey Clark and Ossama Hamdan, the foreign representative of Hamas, as well as Conservative peer Lord Sheikh and Liberal Democrat Lord Phillips.
What was the wreath-laying?
During the trip, Mr Corbyn took up an invitation to join a delegation paying respects to those killed in a 1985 Israeli bombing of the Palestinian Liberation Organisation (PLO) headquarters in Tunis.
Photographs suggest a wreath was laid at the base of a large statue erected in memory of the victims and that Mr Corbyn was next to the memorial, mostly in the background, not holding a wreath.
The delegation then seems to have moved on to a cemetery three miles (5km) away, which houses a monument to those killed in the attack.
It also includes graves of people accused of having links to the 1972 Munich massacre, when Palestinian group Black September killed 11 hostages from the Israeli Olympic team and a West German police officer.
The Palestinian Embassy in Tunisia's Facebook page described the event as a memorial to the 1985 attack, making no mention of the controversial graves.
Among those present were high-level Palestinian figures including Fatah Central Committee member Azzam al-Ahmad, Maher Taher of the Popular Front for the Liberation of Palestine and Palestinian ambassador to Tunisia, Salman el-Herfi.
The layout of the cemetery
See how the memorials are arranged, in this video from the BBC's Rana Jawad.
Did Mr Corbyn lay a wreath?
Yes. Mr Corbyn has confirmed he did lay a wreath, saying this was for those who died in the 1985 bombing.
But critics have pointed out that a photograph from the event appears to show him standing opposite the graves of Atef Bseiso and Salah Khalaf, two senior PLO officers who were accused of links to the Munich attack and were assassinated.
The Daily Mail, which first published the image, quoted Ilana Romano, the widow of weightlifter Yossef who was killed at Munich, saying: "To go to the grave of a person behind the killing of 11 athletes, he should be ashamed and apologise."
There is no photograph of Mr Corbyn laying the wreath at this spot. A memorial for the 1985 air strike is feet away.
The BBC's Rana Jawad, who has visited the cemetery, said she understood the area next to the pair's graves to be the spot where dignitaries and diplomats gather every year to remember victims of the 1985 bombing, as well as senior PLO members.
The cemetery signs, and markings on the graves and memorial, are in Arabic.
Mr Corbyn's spokesman has said he did not take part in laying wreaths on the men's graves. A Labour spokesman also told the Guardian the two men had denied having links with the terrorist group, and that the PLO also disputed this.
What has Mr Corbyn said?
Since the controversy first hit the headlines during the 2017 general election campaign, Mr Corbyn has repeatedly said he was in Tunisia to promote peace in the Middle East and was commemorating victims of the 1985 Israeli bombing.
Attention has focused on a 2014 Morning Star article, in which he wrote that "wreaths were laid at the graves of those who died on that day and on the graves of others killed by Mossad agents in Paris in 1991".
Mossad is the Israeli intelligence agency. Mr Corbyn has not confirmed precisely who he was referring to in this article.
Atef Bseiso was killed in Paris in 1992, and Salah Khalaf was assassinated in Tunisia in 1991.
On Monday, after photographs from the event emerged in the press, Mr Corbyn was asked to clarify his involvement. He said he had been at the event as part of a conference which was "all about the search for peace".
Asked about his Morning Star reference to wreaths for "others killed by Mossad", he said he was present but added: "I don't think I was actually involved in it".
On Tuesday, when pressed further about the pictures of him holding a wreath, he said: "I laid one wreath along with many other people, as I have said, in memory of all those who died in the awful attack in 1985, which, as I keep repeating, was condemned by the whole world."
Israel's Prime Minister Benjamin Netanyahu has attacked Mr Corbyn, accusing him of laying a wreath "on the graves of the terrorist who perpetrated the Munich massacre" and saying he deserved "unequivocal condemnation".
Mr Corbyn said his claim was "false" and hit back at the Israeli prime minister, condemning his government's treatment of Palestinian protesters.
The Labour leader has also been criticised from within his own party by MP Luciana Berger, who tweeted: "Being 'present' is the same as being involved.
"When I attend a memorial, my presence alone, whether I lay a wreath or not, demonstrates my association and support. There can also never be a 'fitting memorial' for terrorists. Where is the apology?"
The Conservatives have sought to capitalise on the row, with Home Secretary Sajid Javid suggesting Mr Corbyn should resign.
But Mr Corbyn's supporters defended him.
Shadow chancellor John McDonnell said Mr Corbyn had "devoted his life to promoting justice and peace" which involved "meeting people who haven't shared those ideals".
Backbench MP Chris Williamson said the Labour leader's "enemies" were using the story to "destabilise" him, accusing them of "indulging in egregious smears".