Jack Straw made two crucial mistakes in his dealings with Tony Blair: one involved the prime minister's relationship with Gordon Brown and the other Iran. Mr Straw has said repeatedly that it is "inconceivable" that there will be a military strike on Iran and last month dismissed as "nuts" a report that George Bush was keeping on the table the option of using tactical nuclear weapons against Tehran's nuclear plants.

But Mr Blair, who sees Iran as the world's biggest threat, does not agree with his former foreign secretary. The prime minister argues that, at the very least, nothing should be ruled out in order to keep Iran guessing. Downing Street phoned the Foreign Office several times to suggest Mr Straw stop going on the BBC Today programme and ruling it out so categorically.

His fate was sealed when the White House called Mr Blair and asked why the foreign secretary kept saying these things. In any case, Mr Straw had boxed himself in on Iran to the extent that he would have had to resign if a military strike became a reality.

Mr Blair was also irritated by what he saw as Mr Straw's opportunism in shifting his loyalty towards Mr Brown with unseemly haste in expectation that he would be the next prime minister.

Iran is one of the issues consuming the Foreign Office and Downing Street. Iraq is the other, and between them they will take up most of Margaret Beckett's time. Mrs Beckett will fly to New York on Monday for a meeting with Condoleezza Rice, the US secretary of state, with whom Mr Straw had established a close rapport, and counterparts from France, Germany, Russia and China to discuss a new UN security council resolution on Iran.

Among the first questions the media will ask is whether she too regards the military option as "inconceivable".

The Foreign Office has taken a battering under Labour. Part of its portfolio, international development, was hived off in 1997 to become a separate department, with its own cabinet minister. And now Mr Blair has created a twin power structure in the Foreign Office by appointing a former cabinet member to what had previously been a relatively junior post, minister for Europe.

Despite the importance placed on relations with Washington, the bulk of British trade is with Europe as is the country's political future. Britain's presidency of the EU, though there was a deal in the end, was fraught.

There is a potential for friction in making the European post more high-profile, but the personalities of Mrs Beckett and Geoff Hoon, both government loyalists, do not suggest conflict.