George Bush told Tony Blair shortly before the invasion of Iraq that he intended to target other countries, including Saudi Arabia, which, he implied, planned to acquire weapons of mass destruction.

Mr Bush said he "wanted to go beyond Iraq in dealing with WMD proliferation, mentioning in particular Saudi Arabia, Iran, North Korea, and Pakistan," according to a note of a telephone conversation between the two men on January 30 2003.

The note is quoted in the US edition, published next week, of Lawless World, America and the Making and Breaking of Global Rules, by the British international lawyer Philippe Sands. The memo was drawn up by one of the prime minister's foreign policy advisers in Downing Street and passed to the Foreign Office, according to Mr Sands.

It is not surprising that Mr Bush referred to Iran and North Korea, or even Pakistan - at the time suspected of spreading nuclear know-how, but now one of America's closest allies in the "war on terror". What is significant is the mention of Saudi Arabia.

In Washington, the neo-cons in particular were hostile to the Saudi royal family and did not think they were doing enough to quell Islamist extremists - 15 of the 19 September 11 attackers were Saudis. But the Bush administration did not in public express concern about any Saudi nuclear ambitions.

In September 2003, the Guardian reported that Saudi Arabia had embarked on a strategic review that included acquiring nuclear weapons. Until then, the assumption in Washington was that Saudi Arabia was content to remain under the US nuclear umbrella despite the worsening relationship between Riyadh and Washington.

It is not clear how Mr Blair responded to Mr Bush's remarks during the telephone conversation, which took place on the eve of a trip to Washington for talks with the US president.

In his book, Blair's Wars, John Kampfner says that at the meeting the two leaders "agreed to concentrate not just on Iraq ... but also the Middle East". But that was taken to be a reference to Palestine. Mr Blair wanted Mr Bush to express concern about the plight of the Palestinians to appease the Labour party.

Mr Blair at the time was careful to avoid any suggestion that the Bush administration intended to target other countries after the invasion of Iraq. However, for the first time he suggested there were links between Saddam Hussein and al-Qaida.

After the invasion, Washington adopted a calmer approach towards Iran, leaving it to Britain, France, and Germany to pursue a diplomatic course.

Despite hard evidence that Pakistan was deeply involved in exporting nuclear technology, the Bush administration embraced President Pervez Musharraf as an ally against al-Qaida. Washington's relations with Saudi Arabia remain cool. Mr Sands does not shed further light on the issue.