The international social networking site Twitter has provoked fury after refusing to act against a user responsible for a stream of death threats and antisemitic abuse. Shereef Abdallah, who advertises himself as an Ed Miliband lookalike and claims to have worked on the Labour leader's election camp, began by targeting people identified as supporters of Tony Blair. He has since widened his online attacks to anyone who challenges him.

As the violence of his language escalated, Mr Abdullah turned his fire on a young British Israeli woman who had criticised his description of his opponents as Nazis. Using often explicit pornographic language Mr Abdullah made direct threats such as: "I will hunt you down & fight you min by min. hr by hr. day by day. wk by wk. month by month. year by year for the rest of yr life."

Mr Abdallah, who has worked as a volunteer at the office of Glenda Jackson and blogged for the Labour Party affiliate organisation Young Fabians, started his campaign with attacks on a young woman who runs the Blairite blog Julie's Think Tank.

The JC has been advised not to reveal the identity of the women involved and told that Mr Adbdallah is at risk of turning his violent, often sexual fantasies into real acts of violence. Many of his tweets end with the sign off R.I.P.

The Probation Officers union Napo, which is running a campaign to toughen up the laws on stalking, referred the tweets to a psychological profiler who helps the police identify violent offenders. She concluded that there was a 30 per cent chance that Mr Abdullah would turn his threats into real acts of violence.

The police have now taken up the case after a former Labour Party press officer intervened only to be threatened with having his throat cut.

However, when a Twitter user known as @blairsupporter reported the abuse received from Mr Abdallah, they received the following reply: "We have investigated the reported account and have found that it's not in violation of the Twitter Rules at this time. We have a policy against violent threats, but the content of this account lacks the specificity to meet the criteria of an actionable threat." A spokeswoman from Twitter's "Trust and Safety" department suggested contacting dealing with the matter locally. She continued: "Websites do not have the ability to investigate and assess a threat, bring charges or prosecute individuals."

A recent federal court ruling found that threats issued by a cyber-stalker to a female Buddhist leader in Maryland were covered by the first amendment which protects freedom of speech. Judge Roger W. Titus found that Twitter and blogs were equivalent to an advertising billboard, which "does not communicate, except to those who voluntarily choose to read what is posted on it."

Community Security Trust spokesman Mark Gardner said Twitter's position set a dangerous precedent:

"These tweets are a clear example of racist intimidation and threats. We cannot have a situation where modern media is exempt from basic legal protocol."

Harry Fletcher, who is running Napo's campaign on stalking law reform said: "Cyber stalking is a serious crime that can lead to violence and even homicide. The threat to these women is real, frightening and could escalate. Social media companies need to wake up to their responsibilities and deal with this as a matter of urgency".

John Mann, chair of the All Party Parliamenteray Group Against Antisemitism said; "It remains a serious concern that individuals continue to peddle hate online, particularly through social media. This highlights the ever-emerging problem with Twitter and it is very troubling that they refuse to act." He said it was essential to build up the number of successful prosecutions.

Jennifer Perry, cyber-stalking spokeswoman for the Network for Surviving Stalking called on the Information Commissioner to intervene: "Unfortunately, the whole internet industry has a complete disregard for the safety of its users. Unless there is a financial incentive or government action this will not change."

Often writing obsessively during the early hours of the morning, Mr Abdallah, who uses the names @Sheik74k and @LFCSheikKD on Twitter, bombarded the young woman who runs Julie's Think Tank with messages such as: "Your nightmare is just starting… it will only get worse for you every day 24/7 till you leave twitter", "Zionists can't save you… the only way to stop it is to leave twitter. Racist tory anti Islamic scum", "I fear nothing not even death so will fight you and your ilk to my last breath.". Most chilling of all was the message: "… are you still alive FFS? You will be next after I end [name of another female target removed]…. that's a promise. sleep tight". The young blogger became seriously concerned when Mr Abdullah turned up at her university.

However, the abuse from Mr Abdallah reserved for the young Jewish woman he targeted descended into direct threats of murder in capital letters: "I WILL BEAT YOU TO DEATH… R.I.P; THIS IS NOT A THREAT… THAT IS A PROMISE."

A criminal profiler consulted by Napo concluded that Mr Abdallah constituted a threat for the following reasons: he was investing disproportionate levels of time and energy into his stalking activities; he showed a high degree of sexual repression; his threatening behaviour was escalating; he had made specific threats of violence and moved from his fantasy world to real life by following one of his targets to her university.

The Fabian Society said Mr Abdallah had falsely claimed to be a member if the Young Fabians when he posted as a blogger on the organisation's website. His post has subsequently been removed.

Mr Abdallah refused to answer the JC's questions about his activities despite several approaches.

● A Jerusalem-based lawyer has claimed that Twitter is providing services to groups such as Hizbollah.

Nitsana Darshan-Leitner, who specialises in cases against terror groups, said that the company could be violating US law.

Although Hizbollah does not have an official Twitter account, that of the Al-Manar TV network, which Hizbollah controls, has 7,500 followers.

● Twitter users in Israel are only able to select "Palestinian Territories" when listing their location.

Users can choose a location using their mobile phones. But anyone wanting to choose Israel finds a drop-down menu which offers only "Palestinian Territories".

Twitter says: "We list countries where we've launched SMS with at least one operator. We've been in touch with operators in Israel but haven't yet been able to launch SMS there. So it's not included on the list."