Elizabeth Wilmhurst, 54, deputy legal adviser, is understood to be unhappy with the government's official line that it has sufficient basis for war under UN resolutions. Ms Wilmhurst has been a legal adviser at the Foreign Office for 30 years, and deputy legal officer since 1997.

Her resignation will be an embarrassment to Tony Blair as well as to Mr Straw and raises new doubts about the legal basis for the war. It will encourage anti-war MPs to renew pressure on the attorney general, Lord Goldsmith, to publish in full his legal advice to the government.

The Foreign Office was reluctant to discuss Ms Wilmhurst's departure. A spokesman said: "A legal adviser has decided to leave over the last few days." Asked the reason, he said: "That is a matter for them."

After a week of reported unease within the government about the legality of going to war without a second UN resolution, Lord Goldsmith on Monday published a condensed version of his advice to Mr Blair. But anti-war MPs and many lawyers suspect the full version may be more evenly balanced.

Concern about the legal advice was expressed this week by two former Foreign Office legal advisers. In a letter to the Times, Sir Franklin Berman, legal adviser from 1991-99, and Sir Arthur Watts, legal adviser from 1987-91, expressed regret that the search for a second resolution had been abandoned. They said the onus was on the government to account "for their actions to the international community in whose name they claim to act".