In his first major foreign policy speech since becoming Prime Minister, Mr Brown called Iran's nuclear ambitions the "greatest immediate threat" to non-proliferation.

The International Atomic Energy Agency (IAEA) and the European Union (EU) are looking into Iran's nuclear program and Mr Brown says Britain will act if he is not satisfied with the outcome of their inquiries.

"We will lead in seeking tougher sanctions, both of the UN and the European Union, including on oil and gas investment," he said.

"The financial sector and Iraq should be in no doubt about the seriousness of our purpose." © 2007 Australian Broadcasting Corporation