Mr Straw’s warning comes as the five permanent members of the UN Security Council – the UK, US, China, France and Russia – prepare to try to end weeks of wrangling on Friday.
The foreign secretary stressed the UK was “completely committed” to using the UN if that was successful.
But America and the US reserved the right to go it alone if military action was blocked and if Iraq was in “flagrant breach” of UN resolutions, he said.
Ending the wrangling
The US says it will soon table a draft resolution to the Security Council aimed at strengthening the weapons inspections regime.
US Secretary of State Colin Powell said the resolution would make it clear Iraq had been “in violation” of UN resolutions for a long time.
And it would make clear there would have to be “consequences for continued violation”, said Mr Powell.
Russia’s Foreign Minister, Igor Ivanov, said he believed there were “favourable conditions” for the council to agree a resolution that quickly restarted weapons inspections.
France is seen as isolated among the key five council members as it continues to insist military action should only come after a second resolution is passed.
Speaking on BBC Radio 4’s Today programme, Mr Straw increased the pressure on the other council members to accept America’s proposals.
“We reserve the right to act within international law in respect of the use of force which may or may not be covered by a new resolution,” he said.
“It is entirely appropriate for America, as for us, to reserve their position if the United Nations does not meet its responsibilities.
“We are completely committed to the United Nations route if that is successful.
“If, for example, we end up being vetoed on statements which are as plain as daylight that Iraq is in flagrant breach of United Nations resolutions, then of course we are in a different situations.”
Those comments are likely to increase criticism from those who say America is planning to attack Iraq regardless of what the UN decides.
UK ministers have stressed their prime objective is dealing with Iraq’s alleged build-up of weapons of mass destruction.
They say they would be delighted to see Saddam Hussein’s government toppled in Baghdad but, unlike some US figures, argue it should not be the aim of their policy.
Iraq’s latest offer to readmit weapons inspectors has not proved enough to satisfy the US and UK Governments.