UK Labour Party leader Jeremy Corbyn has reportedly declined to attend a dinner in London this week with Prime Minister Benjamin Netanyahu celebrating the 100th anniversary of the Balfour Declaration, which pledged Britain’s support for a Jewish national home.

Although no reason was given for Corbyn’s decision to skip the event, which was reported by the Sunday Times, the Labour chief has long been a staunch Israel critic and has been dogged by accusations of tolerating anti-Semitism in his party’s ranks.

In his absence, Corbyn has asked Shadow Foreign Secretary Emily Thornberry to attend the dinner, according to the report, recalling a similar incident last month when he dispatched Thornberry to a Labour Friends of Israel event in his stead.

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Thornberry said at the time that Corbyn was unable to attend as he had to prepare for a speech, although he was later spotted at a separate party event that evening.

While declining to comment on Corbyn’s decision specifically, Israel’s ambassador to the United Kingdom said opponents of the Balfour Declaration are “extremists.”

“Those who oppose the Balfour Declaration are exposing themselves for the extremists they are,” Mark Regev told the Times.

Regev also said a “vocal minority” of students and professors in the UK continue to oppose Israel’s existence 70 years after the country was founded, while adding that “Britain was on the right side of history” in its support for a Jewish state.

In the Balfour Declaration, which was issued on November 2, 1917, then-UK foreign secretary Arthur Balfour told British Jewish leader Lord Walter Rothschild that His Majesty’s government “view with favour the establishment in Palestine of a national home for the Jewish people.”

Ahead of the centennial, British Prime Minister Theresa May said last week the UK would celebrate “with pride” its role in the creation of the State of Israel and upcoming 100th anniversary of the declaration.

While Israel and the UK have geared up to mark the anniversary, Palestinian officials in recent weeks have stepped up calls for London to retract its support for a Jewish homeland in the area of the former British mandate, arguing that the land didn’t belong to Britain and that it therefore had no right to promise it to the Zionist movement.

“Palestinians need the UK to first and foremost recognize its historic responsibility and apologize,” Palestine Liberation Organization Secretary-General Saeb Erekat wrote in a piece for Newsweek.

He also called on Britain to recognize Palestinian statehood and “acknowledge its responsibility and commit to protecting and advancing the political, civil and national rights of the Palestinian people.”

Raphael Ahren contributed to this report.