He also called for a review of the 1949 Geneva Conventions governing the treatment of prisoners of war.

International law on warfare was drawn up with conflicts between countries in mind, and without changes, would be inadequate to cope with terror groups willing and able to kill on a large scale, Reid said.

"We risk trying to avoid 21st century conflict with 20th century rules which, when they were devised, did not contemplate the type of enemy which is now extant," he said.

"The laws of the 20th century placed constraints on us all which enhanced peace and protected liberty. We must ask ourselves whether, as the new century begins, they will do the same."

Reid said the world was facing a threat from terror groups who were unconstrained by morality or adherence to conventions, and which were known to be seeking to acquire weapons of mass destruction.

"We now have to cope with a deliberate regression towards barbaric terrorism by our opponents," he said.

"The legal constraints upon us have to be set against an enemy that adheres to no constraints whatsoever, but an enemy so swift to insist that we do in every particular, and that makes life very difficult for the forces of democracy."

Reid said the spread of weapons of mass destruction posed new questions about the circumstances in which it would be right to mount a pre-emptive strike.

"Hopefully, we would learn of any such threat before any atrocities had been committed. I believe we would have strong legal grounds to take action to protect ourselves against attack. I also suspect that others would disagree."

He made clear he was hoping to stimulate a wider debate.

"What I would like us to explore is to what extent we could impose upon non-state actors the same obligations to civilised conduct, even in warfare, which we apply to ourselves," he said.

Reid was speaking in London to the Royal United Services Institute, Britain's leading professional forum concerned with defence and security.

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