Teams of British military lawyers have been attached to air force and army headquarters and to units on the ground to scrutinise targets, said the sources.
The government is acutely aware that opposition to the war would be fuelled by civilian deaths. It has vowed that while there will be waves of air strikes unprecedented in their intensity, they will be carried out by "smart" bombs.
The sources said under the law of armed conflict, only military objectives and combatants can be targeted. But "something that is normally civilian in use that is being used for military purposes may be a legitimate target", they added.
Under the principle of proportionality, "the expected harm" to civilians "must not be excessive when set against the direct and concrete military advantage anticipated from the attack".
An attack expected to cause civilian deaths would be a crime only if it were "clearly excessive in relation to the anticipated overall military advantage".
Officials said another problem military sources call a potential "nightmare" would be the use of human shields or the deliberate placing of military equipment in civilian locations such as schools or mosques.
Such action would also be illegal, defence sources said yesterday. Any decision to attack such targets would have to have a "very strong" military advantage.
Under international law, they said, even military targets would not always be legitimate. The destruction would have to be judged as having a "definite military advantage in the context of the campaign".
Under the law of armed conflict, Iraq's many cultural and historical sites must be given special protection, as must medical facilities, agricultural areas and drinking water. Attacks causing widespread environmental damage are outlawed.
British officials said yesterday that UK commanders would in principle be able to exercise a veto against targeting orders made by their US superiors by appealing to the attorney general or their political masters in London. They added that this was unlikely.
But military sources yesterday expressed concern about the possibility of "friendly fire" - American or British forces firing on each other.
They also spelt out the possibility of black propaganda used by Saddam Hussein, including television pictures of Iraqi soldiers dressed in British or US uniforms carrying out attacks on Iraqi civilians.
But the sources admitted it was a "big step" to move from planning such measures to actually carrying them out.
Defence officials said an Iraqi propaganda campaign was likely to be aimed at France, Russia and China - which they said were seen as the "easier ones".
Barbara Stocking, director of Oxfam, said last night: "The burden of proof will lie with governments to show they have not breached international humanitarian law.
"If they hit targets essential to civilian life such as electricity plants that power water and sewage systems, we will want to know what the basis for this was and the legal advice they relied on."
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