Barack Obama should brace himself for a terrorist attack using nuclear or more likely biological weapons before 2013, a bi-partisan commission appointed by the United States Congress will warn on Wednesday.

"Our margin of safety is shrinking, not growing," states the report from the Commission on the Prevention of Weapons of Mass Destruction Proliferation and Terrorism.

Vice President-elect Joe Biden is being briefed on the study which based its findings on evidence from more than 250 military, political and academic experts.

Key recommendations for Mr Obama include bolstering preparations for germ warfare, as well as appointing an official on the National Security Council to coordinate intelligence and foreign policy on combating the spread of WMD.

Former Senator Bob Graham, one of the co-chairman of the commission, said yesterday: "Terrorists have made it quite clear that the United States of America is their number one target."

He pointed to the carnage wrought by just a handful of gun men in India last week, saying: "If those people had had access to a biological or nuclear weapon they would have multiplied by orders of magnitude the deaths they could have inflicted."

Al-Qaeda remains the only terrorist group judged to be actively intent on a nuclear attack against the US. Although the report says the organisation is not yet capable of building or obtaining such a weapon, that could change if it recruited a nuclear scientist.

The report says the biggest threat comes from Pakistan. Commission members were forced to cancel their trip to Islamabad this autumn after their hotel was blown up by terrorists just hours before their arrival.

They are particularly concerned about biological weapons, because civilian laboratories with potentially dangerous pathogens are not subject to the same security as nuclear sites.

Mr Graham said anthrax remains the most likely biological weapon, but contagious diseases are also becoming threats. The flu virus that that killed 40 million people at the beginning of the 20th century has been recreated in scientific labs and there remains no inoculation to protect against it if is released.

"The United States should be less concerned that terrorists will become biologists and far more concerned that biologists will become terrorists," the report says.

President Bush has repeatedly warned about the prospect of a terror attack during the transition or early in the next administration. And Mr Biden, speaking shortly before last month's election also raised fears, saying: "Remember I said it standing here, we’re gonna have an international crisis, a generated crisis, to test the mettle of this guy.”

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