"This afternoon Keir Starmer asked Rebecca Long-Bailey to step down from the shadow cabinet," a spokesman for the Labour leader said.
"The article Rebecca shared earlier today contained an antisemitic conspiracy theory.
"As Leader of the Labour Party, Keir has been clear that restoring trust with the Jewish community is a number one priority.
"Antisemitism takes many different forms and it is important that we all are vigilant against it."
Ms Long-Bailey, who stood in the race to replace Jeremy Corbyn, was chosen by Sir Keir to be shadow education secretary when he became leader in April.
Sky News understands a campaign group of left-wing Labour MPs have requested a meeting with Sir Keir Starmer about the dismissal but their request was declined.
Ms Long-Bailey's sacking comes after she shared, on Twitter, an interview with the actress Maxine Peake, describing her as an "absolute diamond".
In the interview, Ms Peake claimed the US police linked to the killing of George Floyd had learned their tactics from the Israeli secret services.
After attracting criticism for her tweet, Ms Long-Bailey said she "retweeted Maxine Peake's article because of her significant achievements and because the thrust of her argument is to stay in the Labour Party".
She added: "It wasn't intended to be an endorsement of all aspects of the article."
In a series of messages after her sacking was announced, Ms Long-Bailey said she shared the article from "my constituent and stalwart Labour Party supporter Maxine Peake" because "its main thrust was anger with the Conservative government's handling of the current emergency and a call for Labour Party unity".
She added: "These sentiments are shared by everyone in our movement and millions of people in our country.
"I learned that many people were concerned by references to international sharing of training and restraint techniques between police and security forces.
"In no way was my retweet an intention to endorse every part of that article."
Ms Long-Bailey said she wanted to "acknowledge these concerns and duly issued a clarification of my retweet", which was agreed in advance with Sir Keir's office.
"But after posting I was subsequently instructed to take both this agreed clarification and my original retweet down," she said.
"I could not do this in good conscience without the issuing of a press statement of clarification.
"I had asked to discuss these matters with Keir before agreeing what further action to take, but sadly he had already made his decision."
Ms Long-Bailey, who became MP for Salford and Eccles in 2015, said she would "continue to support the Labour Party in parliament under Keir Starmer's leadership, to represent the people of Salford and Eccles and work towards a more equal, peaceful and sustainable world".
Labour MP Dame Margaret Hodge, who is Jewish, tweeted: "This is what a change in culture looks like. This is what zero tolerance looks like. This is what rebuilding trust with the Jewish community looks like."
Marie van der Zyl, president of the Board of Deputies of British Jews, thanked Sir Keir for his "swift action" and said he was "backing his words with actions on antisemitism".
Jonathan Goldstein, chair of the Jewish Leadership Council, said Sir Keir's "decisive leadership and firm action" shows "he understands the severity and harm that antisemitic conspiracies do to our politics".
John Mann, the government's antisemitism adviser and a former Labour MP, added Sir Keir had done "the decent thing" and "the Jewish community will be significantly reassured because they wanted to see action - not just words".
But John McDonnell, who was shadow chancellor under Mr Corbyn, made clear that he disagreed with the decision.
"Throughout discussion of antisemitism it's always been said criticism of practices of Israeli state is not antisemitic," he said.
"I don't believe therefore that this article is or [email protected]_Bailey should've been sacked. I stand in solidarity with her."
Maxine Peake clarified her own reported comments after the row.
"I was inaccurate in my assumption of American police training and its sources," she tweeted.
"I find racism and antisemitism abhorrent and I in no way wished, nor intended, to add fodder to any views of the contrary."