Sheikh Omar Bakri Mohammed, who leads Al-Muhajiroun, said he was aware of "many" threats of retaliation from overseas radicals if action is taken against Saddam Hussein's regime.
Speaking to BBC Radio 4's Today programme he called on Muslims living in Britain to be "patient" and not to get involved in any violent plots.
Sheikh Mohammed said he wanted to warn the British Government and people against attacking Iraq because "the consequences of playing with fire is always either you burn yourself or cause other problems".
'Worried'
The fundamentalist said he condemned all violence, but described the 11 September suicide hijackings as "a direct response to the evil American policy in the Muslim world".
And asked if he knew of any specific threats against the UK he said: "If you are in my position and listening to the talk and discussion in different rooms on the Internet and aware about Islamic activities, yes you will be worried, because I receive many of these type of threats."
Sheikh Mohammed said he was aware of many "freelance" Muslim radicals who believe they are at war with the West and are prepared to take action.
He added: "If they (the West) are attacking Muslims in Iraq, maybe some Muslims abroad will retaliate and we don't want to be caught in the middle."
Radical opinions
Sheikh Mohammed said action against Iraq would not only be dangerous for the stability of the Middle East, but also for Muslims living in Britain.
He said British Muslims were at risk of being accused of supporting terrorism at the same time as being the targets of terrorists, because they are living in the West.
Last month Sheikh Mohammed joined other fundamentalist Muslims at a conference in London, to discuss the War on Terror.
Mainstream leaders steered clear of the meeting, saying that the radical opinions of the speakers did not represent the views of the majority of Britain's 1.5 million Muslims.