A British Airways Boeing 777 jetliner lands at Chicago's O'Hare International Airport in this April, 2003 file photo. (Chris Kilroy/View Full Size)

The pair smuggled plastic explosives into Israel from Jordan inside copies of the Koran, the paper reported Israel's Defence Minister Shaoul Mofaz as saying.

Israeli officials investigating how Asif Mohammed Hanif and Omar Khan Sharif were able to penetrate tight border controls suggested that they could have used a new kind of explosive that is more difficult to detect, according to the Telegraph.

The prospect of such an explosive in the hands of terrorists raises worrying questions for airline security around the world. Western intelligence officials will be seeking confirmation that the claim is not an attempt by Israeli security to cover up an embarrassing lapse, the paper added.

Israeli newspapers have dropped hints about the use of "an unfamiliar substance" in the attack on Mike's Place seafront bar last Wednesday , in which three Israelis and a suicide bomber were killed and dozens injured.

Mofaz told the Israeli cabinet that the Britons probably obtained the explosives in Jordan or Syria and entered Israel from Jordan several weeks ago.

Assif Mohammed Hanif blew himself up in a bomb attack in Israel on Wednesday while his suspected accomplice, Omar Khan Sharif, is said to be on the run.

Scotland Yard arrested six people in connection with the deadly attack, including Sharif's wife, two sisters and other family members.

All six were arrested under section 41 of the Terrorism Act 2000 which relates to those suspected of being involved in the commission, preparation or instigation of acts of terrorism.

British security chiefs knew that two Britons linked to a suicide bomb attack in Israel last week were connected to Islamic extremists, but decided they were not potential terrorists, a London newspaper said Sunday.