Hassan Butt, a self-styled recruiter of British volunteers for the Islamic holy war, said he had been approached by the men for guidance on how to carry out "martyrdom operations" in Israel.

The warning came after last week's suicide bombing at a bar in Tel Aviv which killed three people. The attack by Asif Mohammed Hanif, from Hounslow, west London, and Omar Khan Sharif, from Derby, was the first suicide strike by British citizens. Hanif blew himself up but Sharif fled when his bomb failed to detonate. He is still on the run.

Security sources said Sharif was known to police in Derby. "He was a known Islamic activist and there were reports that he was involved in handing out fundamentalist leaflets," said one. However, police denied there was any reason to suspect he would act as a suicide bomber. Scotland Yard's anti-terrorist branch is questioning six people it suspects might have been linked to the blast.

This weekend Butt said he knew both British bombers but refused to elaborate. He claimed they were just the first of a significant number of Britons who were preparing to conduct similar attacks. "The number is getting close to 50. They are aged 17 to their late thirties. They are contacting me about organisation."

He said he believed "about 20" of them were "absolutely serious". Most of the men were living in Britain but had family roots in Pakistan, India and Bangladesh. "They are waiting for the right time, the right people. You don't just do it as individuals, you do it as an organisation. It's about screening them, testing them, making sure they are sincere. Then, when it's right, believe me, they'll all be used."

In the past, security officials have been sceptical about Butt's claims. However, last November he was arrested and questioned by Special Branch detectives who believed he had information relating to terror attacks. At about the same time he warned that he knew British-based fundamentalists were preparing to carry out suicide attacks abroad. He was also proved correct in his claims that scores of Britons had gone to fight with the Taliban in Afghanistan, introducing some of them to journalists.

One of the six being questioned by police yesterday was Tahira Sharif, the failed bomber's 26-year-old wife. Detectives want to know if she had any knowledge of the attack and are examining mobile phone records, personal computers and religious literature seized at their Derby home. Other members of Sharif's family are also being questioned. One source said: "The family are victims as well as suspects. There will be parts of the family that will be genuinely gobsmacked by all this and there will be parts that probably won't."

Senior security sources last night dismissed reports that MI5 had been aware of the bombers' extremist activities and decided they were not potential terrorists. "The security services did not know who these people were until they let off the bombs," a senior Whitehall official said.

MI5 believes the attacks were planned abroad. Sharif and Hanif attended Arabic language courses at Damascus University. A student there said: "Asif was a cheerful guy, we knew him better than Omar. He was around for a couple of months and then disappeared. It was a surprise. No one knew until they saw the news."

Security sources believe the two men may have been indoctrinated by radical clerics there who introduced them to terrorists from Hamas. The bombers may have also received financial backing, spiritual support and an introduction to contacts from supporters of Al-Muhajiroun, a fanatical group that wants to turn Britain into an Islamic state.

The group yesterday denied the men were members but Omar Bakri Mohammed, its leader, said he had acted as spiritual adviser to them. Bakri said Sharif had attended six two-hour sessions in Derby, with the last on April 14, just over a fortnight before the bombing.

Sharif had asked Bakri to become his spiritual "companion", a status accorded to extreme fundamentalists who are willing to be imprisoned and to suffer for their faith up to death. Bakri said he declined the offer because he was too busy. He denies encouraging Sharif to carry out the attack.

Officers from Mossad, the Israeli intelligence service, are probing links between the bombers and Al-Muhajiroun. A senior detective said: "Al-Muhajiroun says it just sends people out there for religious instruction. The question is: at what point does spiritual advice become incitement to murder?"

Israeli security sources said the men used an explosive not previously employed in suicide attacks. They claimed they were trained in Syria by Hezbollah and instructed by the head of Hamas's overseas operations.

Additional reporting: Uzi Mahnaimi

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