None of the United Nations resolutions demanding that Iraq disarms would justify an attack, the 16 eminent figures said in the letter, quoted in Friday's Guardian newspaper.
It comes as a deeply split Security Council was set to debate a British-US resolution Friday seeking UN backing for war on Iraq, to be preceded by a UN weapons inspectors report.
In the letter, the lawyers said that "on the basis of the information publicly available, there is no justification under international law for the use of military force against Iraq".
Speaking on BBC radio, one of the letter's signatories, Philippe Sands, professor of international law at University College London, argued there was no legal case for war because there was "no basis for individual self-defence and there is no Security Council resolution authorising collective self-defence".
"So without those two basic criteria being set there's no legal basis for the use of force," added Sands, who is a member of the Matrix legal chambers in London to which Cherie Blair, the prime minister's lawyer wife, belongs.
British Foreign Secretary Jack Straw said Tuesday that London feels it has sufficient legal authority to go to war with the United States against Iraq without a new UN resolution.
Straw said that as far as Britain was concerned November's UN Resolution 1441 and others adopted before it were good enough under international law for the United States and Britain to act militarily.
"We are satisfied that we have sufficient legal authority in 1441, back to the originating resolution 660 (in August 1990) and so on, to justify military action against Iraq if they are in further material breach" of UN demands to disarm, he told parliament's foreign affairs committee.
The group of lawyers hit out at Blair's suggestions that he would ignore an "unreasonable veto" from one or more of the five permanent Security Council members if they blocked a fresh United Nations resolution opening the way to war.
"A decision to undertake military action in Iraq without proper Security Council authorisation will seriously undermine the international rule of law," the letter said.
"Of course, even with authorisation, serious questions would remain. A lawful war is not a just, prudent or humanitarian war." Others signatories of the letter include six leading international lawyers from Britain's prestigious Oxford University, three from Cambridge University, and three from the London School of Economics.
Meanwhile a report in The Times said Britain's campaign to win backing for a new UN resolution paving the way to war will falter Friday when chief arms inspector Hans Blix delivers his most positive verdict yet on Iraq's cooperation on disarmament.
The British daily said Blix's report would boost the efforts of France, Germany and Russia to avoid conflict by launching a personal political initiative to give Iraq more time.
The chief arms inspector planned to tell the UN Security Council that Iraq has demonstrated substantive cooperation in a number of areas, such as destroying its al-Samoud 2 missiles and allowing private interviews with scientists, The Times said, without indicating its sources.
Blix would also publish a list of previously classified questions for Iraq that formed the basis of an alternative proposal by France, Germany and Russia to give inspections more time - Sapa-AFP.
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