Labour has sought to distance itself from a series of pro-Jeremy Corbyn Facebook groups that feature anti-Semitic, racist and abusive comments.
A report published in the Sunday Times claims to have uncovered more than 2,000 such messages, including misogynistic and violent posts.
Among the messages was praise for Adolf Hitler and threats to kill Prime Minister Theresa May, the paper said.
A Labour spokesman said the groups are not connected to the party in any way.
The report is the result of a two-month investigation by the Sunday Times into the 20 biggest pro-Corbyn Facebook groups.
Altogether the groups have a membership of around 400,000 people, it said.
Among the members are 12 senior staff working for Mr Corbyn and shadow chancellor John McDonnell, the newspaper claims.
The Times reported that comments in the groups show members advocating violence.
The paper also said it found messages comparing the Conservatives to Nazis and suggesting a journalist should be killed.
Writing in the newspaper, Jewish Labour MP Luciana Berger said she had received an email from someone who claimed to be a party member telling her she should kill herself.
She had also been called "Judas", "a venal piece of detritus", a "Zionazi" and an "absolute parasite."
A spokesman for the Labour Party said: "These groups are not run by the Labour Party or officially connected to the party in any way."
Labour also told the Times that no-one in Mr Corbyn or Mr McDonnell's offices has seen, posted or endorsed anti-Semitic or abusive messages.
The latest allegations mark a deepening in the row over anti-Semitism in the Labour Party which has dogged Mr Corbyn for the past week.
It erupted when Mr Corbyn himself was criticised for opposing the removal of an anti-Semitic mural in east London in 2012.
On Saturday, senior Labour official Christine Shawcroft resigned from the party's ruling body, the National Executive Committee, and was replaced by comedian Eddie Izzard.
It had emerged that she had opposed the suspension of a council candidate accused of denying the Holocaust.
On Friday Mr Corbyn admitted Labour must "do better" over its handling of anti-Semitism.