Labour faced further humiliation tonight after it emerged that three people have been arrested by police investigating allegations of anti-Semitism among party members.

Two men and a woman were detained in the past three weeks on suspicion of publishing or distributing material likely to stir up racial hatred. They were released pending further inquiries.

The police probe was prompted by an internal Labour dossier detailing anti-Semitic social media messages allegedly posted by party members, which was obtained by LBC radio.

Patricia Sheerin, 72, from Wandsworth, south London, was detained on suspicion of publishing or distributing material likely to stir up racial hatred

Two of those arrested were Patricia Sheerin, 72, from Wandsworth, south London, and Roy Smart, 50, from Tunbridge Wells, Kent, it emerged last night. Both allegedly posted about the Holocaust on social media. LBC’s documents included details of 45 cases of alleged anti-Semitism, including one which read: ‘We shall rid the Jews who are a cancer on us all.’

Others included a threat to throw two MPs off the top of a building and one that said a ‘Zionist extremist MP who hates civilised people’ was going to ‘get a good kicking’.

Despite knowing of the threats, Labour did not pass the dossier to police. The radio station handed it to Scotland Yard Commissioner Cressida Dick in September. Last night Jewish groups said the party’s failure to hand over the document was proof it is institutionally anti-Semitic.

Roy Smart (left), 50, from Tunbridge Wells, Kent, allegedly posted about the Holocaust on social media

Euan Philipps, of Labour Against Anti-Semitism, said: ‘We are pleased that anti-Semitic hate crime within the Labour Party is finally being recognised and acted on at a criminal level, and we hope the CPS will proceed to a prosecution.

‘What needs to be addressed is that apparently figures within the Labour Party administration were aware of this evidence and had been sitting on it for months.

‘Their reported failure to even inform their own MPs of the vile… abuse being directed at them (and which possibly threatened their physical security) is a gross and direct dereliction of their and the party’s responsibilities.

‘These failures point yet again to an endemic culture of institutional anti-Semitism within the Labour Party, that we hope the EHRC will soon investigate.’

MP Ian Austin, who quit Labour over the party’s anti-Semitism problem, said: ‘Jeremy Corbyn keeps claiming the Labour Party is taking anti-Semitism seriously, but it’s really shocking to discover they had this information and did nothing with it even though the police now think there are grounds to arrest them.’

It is understood that none of the three detained is still a member of the Labour Party.

The Metropolitan Police said a man in his 50s was arrested in Birmingham on March 7, a man in his 50s was held in Tunbridge Wells on March 14 and a woman in her 70s was arrested in Wandsworth on March 21. All were held on suspicion of publishing or distributing material likely to stir up racial hatred.

Scotland Yard said the contents of the dossier had been examined by specialist officers and the three were arrested, interviewed under caution and released under investigation.

Last week Miss Sheerin’s daughter Kate wrote on Facebook that six or seven officers were searching her mother’s home. Miss Sheerin later wrote that her computer and mobile phone had been seized.

Mr Smart’s wife confirmed yesterday that he had been arrested. He had been due to stand as a Labour candidate in council elections in Tunbridge Wells but is now not doing so.

MP Ian Austin, who quit Labour over the party’s anti-Semitism problem, claimed Jeremy Corbyn wasn't taking the problem seriously

In a joint statement, the Board of Deputies of British Jews, the Community Security Trust and the Jewish Leadership Council said: ‘We await the outcome of the investigations, but ask why Labour had not itself alerted the police.

'We are also concerned as to how many more cases remain unknown and repeat our request, made one year ago to Mr Corbyn, that the party urgently appoint an independent ombudsman on such matters.’

The Labour Party said: ‘We welcome the police investigating these individuals’ alleged crimes.

‘Anti-Semitism has no place in our society and we are committed to challenging and campaigning against it in all its forms.’

n The Conservatives could face a probe by the equalities watchdog into the way they have dealt with claims of Islamophobia.

It emerged last week that 15 Tory councillors had been quietly let back into the party despite posting racist content online.

Last night the Equality and Human Rights Commission said: ‘We have received a complaint and will consider the issues raised.’