Labour's shadow immigration minister has said he is "sincerely sorry" after sharing what he described as "an anti-Semitic conspiracy" on social media.
Manchester Gorton MP Afzal Khan shared the Facebook post in 2015 when he was an MEP for the North West of England.
His apology comes days after Manchester councillor Majid Dar was suspended over alleged anti-Semitic posts online.
Conservative politicians and Jewish groups have called for an inquiry into Mr Khan's conduct.
The post in question features a video of the US comedian Jon Stewart talking about the Israeli prime minister Benjamin Netanyahu.
Underneath the video the text prominently refers to an "Israel-British-Swiss-Rothschilds crime syndicate" and "mass murdering Rothschilds Israeli mafia criminal liars".
Raphi Bloom, co-chair of North West Friends of Israel, said the reference to the Rothschilds was "a well-known anti-Semitic trope about Jews dominating the world".
The post contravened the Labour Party's own definition of anti-Semitism, Mr Bloom said. "I think it's disgraceful and I would like to see action."
After being contacted about the post by the BBC, Mr Khan said "I am mortified and sincerely sorry about this genuine accident.
"I didn't read the text below, which contained an anti-Semitic conspiracy about the Rothschilds. I would never have shared it if I had seen that," he said.
Shaden Jaradat, the Gorton Conservative candidate who campaigned against Mr Khan in 2017, said he believed the post could have influenced voters if known about at the time.
"We're talking about someone who was an MEP for many years, who is in the shadow cabinet," he said.
"If anything, we have to have higher standards for people who are on the front bench and have been in the game for so long,"
Mr Khan's apology also comes two months after the government's equalities watchdog launched an inquiry into the Labour Party over further allegations of anti-Semitism.
The Facebook post was provided to the BBC by a voluntary group of Labour members called Labour Against Anti-Semitism.
Their spokesman, Euan Philipps, called for Mr Khan to be removed from the party, saying the MP had also recently liked another Facebook post claiming Jeremy Corbyn was "the target of a relentless smear campaign, orchestrated by the wealthy establishment".
Mr Khan said of that allegation: "I have always been clear, as has Jeremy Corbyn, that concerns about anti-Semitism within our Party must not be dismissed as smears."
Mr Khan is well-known for his work with different community groups in Manchester and helped found the Muslim Jewish Forum of Greater Manchester.
Its co-chair, Mohammed Amin, said he had known Mr Khan for 20 years and while he was "careless" and showed "poor-judgment", the MP was not an anti-Semite.
Mr Amin said: "My view of Afzal Khan is that he's not anti-Jewish," before adding Mr Khan was just, "very sloppy".
"There are lots of people in the Labour Party who have got themselves in trouble with allegations of anti-Semitism through their sloppiness."
A Labour Party spokeswoman said they could not comment on any individual, but it was understood the national party had not received a complaint against Mr Khan.
Any such complaint would be referred to the Whips office.