Five Muslim community workers have accused MI5 of waging a campaign of blackmail and harassment in an attempt to recruit them as informants.

The men claim they were given a choice of working for the Security Service or face detention and harassment in the UK and overseas.

They have made official complaints to the police, to the body which oversees the work of the Security Service and to their local MP Frank Dobson. Now they have decided to speak publicly about their experiences in the hope that publicity will stop similar tactics being used in the future.

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Intelligence gathered by informers is crucial to stopping further terror outrages, but the men's allegations raise concerns about the coercion of young Muslim men by the Security Service and the damage this does to the gathering of information in the future.

Three of the men say they were detained at foreign airports on the orders of MI5 after leaving Britain on family holidays last year.

After they were sent back to the UK, they were interviewed by MI5 officers who, they say, falsely accused them of links to Islamic extremism. On each occasion the agents said they would lift the travel restrictions and threat of detention in return for their co-operation. When the men refused some of them received what they say were intimidating phone calls and threats.

Two other Muslim men say they were approached by MI5 at their homes after police officers posed as postmen. Each of the five men, aged between 19 and 25, was warned that if he did not help the security services he would be considered a terror suspect. A sixth man was held by MI5 for three hours after returning from his honeymoon in Saudi Arabia. He too claims he was threatened with travel restrictions if he tried to leave the UK.

An agent who gave her name as Katherine is alleged to have made direct threats to Adydarus Elmi, a 25-year-old cinema worker from north London. In one telephone call she rang him at 7am to congratulate him on the birth of his baby girl. His wife was still seven months' pregnant and the couple had expressly told the hospital that they did not want to know the sex of their child.

Mr Elmi further alleges: "Katherine tried to threaten me by saying, and it still runs through my mind now: 'Remember, this won't be the last time we ever meet.' And then during our last conversation she explained: 'If you do not want anything to happen to your family you will co-operate.'"

Madhi Hashi, a 19-year-old care worker from Camden, claims he was held for 16 hours in a cell in Djibouti airport on the orders of MI5. He alleges that when he was returned to the UK on 9 April this year he was met by an MI5 agent who told him his terror suspect status would remain until he agreed to work for the Security Service. He alleges that he was to be given the job of informing on his friends by encouraging them to talk about jihad.

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Mohamed Nur, 25, a community youth worker from north London, claims he was threatened by the Security Service after an agent gained access to his home accompanied by a police officer posing as a postman.

"The MI5 agent said, 'Mohamed if you do not work for us we will tell any foreign country you try to travel to that you are a suspected terrorist.'"

Mohamed Aden, 25, a community youth worker from Camden, was also approached by someone disguised as a postman in August last year. He alleges an agent told him: "We're going to make your travelling harder for you if you don't co-operate."

None of the six men, who work with disadvantaged youths at the Kentish Town Community Organisation (KTCO), has ever been arrested for terrorism or a terrorism-related offence.

They have repeatedly complained about their treatment to the police and to the Investigatory Powers Tribunal, which oversees the work of the Security Services.

In a letter to Lord Justice Mummery, who heads the tribunal, Sharhabeel Lone, the chairman of the KTCO, said: "The only thing these young people have in common is that they studied Arabic abroad and are of Somali origin. They are not involved in any terrorist activity whatsoever, nor have they ever been, and the security services are well aware of this."

Mr Sharhabeel added: "These incidents smack of racism, Islamophobia and all that undermines social cohesion. Threatening British citizens, harassing them in their own country, alienating young people who have committed no crime other than practising a particular faith and being a different colour is a recipe for disaster.

"These disgraceful incidents have undermined 10 years of hard work and severely impacted social cohesion in Camden. Targeting young people that are role models for all young people in our country in such a disparaging way demonstrates a total lack of understanding of on-the-ground reality and can only be counter-productive.

"When people are terrorised by the very same body that is meant to protect them, sowing fear, suspicion and division, we are on a slippery slope to an Orwellian society."

Frank Dobson said: "To identify real suspects from the Muslim communities MI5 must use informers. But it seems that from what I have seen some of their methods may be counter-productive."

Last night MI5 and the police refused to discuss the men's complaints with The Independent. But on its website, MI5 says it is untrue that the Security Service harasses Muslims.

The organisation says: "We do not investigate any individuals on the grounds of ethnicity or religious beliefs. Countering the threat from international terrorists, including those who claim to be acting for Islam, is the Security Service's highest priority.

"We know that attacks are being considered and planned for the UK by al-Qai'da and associated networks. International terrorists in this country threaten us directly through violence and indirectly through supporting violence overseas."

It adds: "Muslims are often themselves the victims of this violence – the series of terrorist attacks in Casablanca in May 2003 and Riyadh in May and November 2003 illustrate this.

"The service also employs staff of all religions, including Muslims. We are committed to recruiting a diverse range of staff from all backgrounds so that we can benefit from their different perspectives and experience."

MI5 and me: Three statements

Mahdi Hashi: 'I told him: this is blackmail'

Last month, 19-year-old Mahdi Hashi arrived at Gatwick airport to take a plane to visit his sick grandmother in Djibouti, but as he was checking in he was stopped by two plainclothes officers. One of the officers identified himself as Richard and said he was working for MI5.

Mr Hashi said: "He warned me not to get on the flight. He said 'Whatever happens to you outside the UK is not our responsibility'. I was absolutely shocked." The agent handed Mr Hashi a piece of paper with his name and telephone contact details and asked him to call him.

"The whole time he tried to make it seem like he was looking after me. And just before I left them at my boarding gate I remember 'Richard' telling me 'It's your choice, mate, to get on that flight but I advise you not to,' and then he winked at me."

When Mr Hashi arrived at Djibouti airport he was stopped at passport control. He was then held in a room for 16 hours before being deported back to the UK. He claims the Somali security officers told him that their orders came from London. More than 24 hours after he first left the UK he arrived back at Heathrow and was detained again.

"I was taken to pick up my luggage and then into a very discreet room. 'Richard' walked in with a Costa bag with food which he said was for me, my breakfast. He said it was them who sent me back because I was a terror suspect." Mr Hashi, a volunteer youth leader at Kentish Town Community Organisation in north London, alleges that the officer made it clear that his "suspect" status and travel restrictions would only be lifted if he agreed to co-operate with MI5. "I told him 'This is blatant blackmail'; he said 'No, it's just proving your innocence. By co-operating with us we know you're not guilty.'

"He said I could go and that he'd like to meet me another time, preferably after [May] Monday Bank Holiday. I looked at him and said 'I don't ever want to see you or hear from you again. You've ruined my holiday, upset my family, and you nearly gave my sick grandmother in Somalia a heart attack'."

Adydarus Elmi: 'MI5 agent threatened my family'

When the 23-year-old cinema worker from north London arrived at Chicago's O'Hare airport with his pregnant wife, they were separated, questioned and deported back to Britain.

Three days later Mr Elmi was contacted on his mobile phone and asked to attend Charing Cross police station to discuss problems he was having with his travel documents. "I met a man and a woman," he said. "She said her name was Katherine and that she worked for MI5. I didn't know what MI5 was."

For two-and-a-half hours Mr Elmi faced questions. "I felt I was being lured into working for MI5." The contact did not stop there. Over the following weeks he claims "Katherine" harassed him with dozens of phone calls.

"She would regularly call my mother's home asking to speak to me," he said. "And she would constantly call my mobile."

In one disturbing call the agent telephoned his home at 7am to congratulate him on the birth of his baby girl. His wife was still seven months pregnant and the couple had expressly told the hospital that they did not want to know the sex of their child.

"Katherine tried to threaten me by saying – and it still runs through my mind now – 'Remember, this won't be the last time we ever meet", and then during our last conversation explained: 'If you do not want anything to happen to your family you will co-operate'."

Mohamed Nur

Mohamed Nur, 25, first came into contact with MI5 early one morning in August 2008 when his doorbell rang. Looking through his spyhole in Camden, north London, he saw a man with a red bag who said he was a postman.

When Mr Nur opened the door the man told him that he was in fact a policeman and that he and his colleague wanted to talk to him. When they sat down the second man produced ID and said that he worked for MI5.

The agent told Mr Nur that they suspected him of being an Islamic extremist. "I immediately said 'And where did you get such an idea?' He replied, 'I am not permitted to discuss our sources'. I said that I have never done anything extreme."

Mr Nur claims he was then threatened by the officer. "The MI5 agent said, 'Mohamed, if you do not work for us we will tell any foreign country you try to travel to that you are a suspected terrorist'."

They asked him what travel plans he had. Mr Nur said he might visit Sweden next year for a football tournament. The agent told him he would contact him within the next three days.

"I am not interested in meeting you ever." Mr Nur replied. As they left, the agent said to at least consider the approach, as it was in his best interests.