In an open letter to 10 Downing Street, published in the Guardian newspaper, a group of 16 academic lawyers have argued that taking action without a new, clear United Nations mandate "will seriously undermine the international rule of law".

This is the full text.

We are teachers of international law. On the basis of the information publicly available, there is no justification under international law for the use of military force against Iraq. The UN charter outlaws the use of force with only two exceptions: individual or collective self-defence in response to an armed attack and action authorised by the security council as a collective response to a threat to the peace, breach of the peace or act of aggression.

There are currently no grounds for a claim to use such force in self-defence. The doctrine of pre-emptive self-defence against an attack that might arise at some hypothetical future time has no basis in international law.

Neither security council resolution 1441 nor any prior resolution authorises the proposed use of force in the present circumstances. Before military action can lawfully be undertaken against Iraq, the security council must have indicated its clearly expressed assent. It has not yet done so. A vetoed resolution could provide no such assent.

The prime minister's assertion that in certain circumstances a veto becomes "unreasonable" and may be disregarded has no basis in international law.

The UK has used its security council veto on 32 occasions since 1945. Any attempt to disregard these votes on the ground that they were "unreasonable" would have been deplored as an unacceptable infringement of the UK's right to exercise a veto under UN charter article 27. A decision to undertake military action in Iraq without proper security council authorisation will seriously undermine the international rule of law. Of course, even with that authorisation, serious questions would remain.

A lawful war is not necessarily a just, prudent or humanitarian war.

Prof Ulf Bernitz, Dr Nicolas Espejo-Yaksic, Agnes Hurwitz, Prof Vaughan Lowe, Dr Ben Saul, Dr Katja Ziegler (University of Oxford), Prof James Crawford, Dr Susan Marks, Dr Roger O'Keefe (University of Cambridge), Prof Christine Chinkin, Dr Gerry Simpson, Deborah Cass (London School of Economics), Dr Matthew Craven (School of Oriental and African Studies), Prof Philippe Sands, Ralph Wilde (University College London), Prof Pierre-Marie Dupuy (University of Paris)