Daniel Ellsberg, who leaked secret Pentagon documents during the Vietnam war, said on Wednesday that he believed the US would attack Iran before 2008 and urged Washington insiders to make new disclosures to prevent a new war.

Ellsberg, who is one of four recipients of this year’s Right Livelihood Award, often dubbed the “Alternative Nobels,” being presented in Stockholm this week, also urged US allies to threaten to withdraw from the Nato alliance if nuclear weapons are used against Iran. “It is more likely than not, in the next two years, that President Bush and Vice President Cheney will direct an attack on Iran,” Ellsberg said at a news conference for the Right Livelihood laureates.

“Such an attack ... might escalate too, to the use of nuclear weapons against underground installations in Iran, within calculable consequences.” But, he added: “Of the various disastrous policies of their administration, this one is the most susceptible to being changed and averted by public pressure.” Ellsberg, 75, was honoured with the prize for leaking the so-called Pentagon Papers, which indicated the US government had deceived the public about whether the Vietnam war could be won and the extent of casualties, and for continuing efforts to expose government deception worldwide.

A former US State Department official, he now called on current Washington insiders to release any classified documents that could sway public opinion against an attack. “Don’t do what I did, don’t wait until the war has started before you tell the truth with documents,” Ellsberg told The Associated Press.

He also said European allies and other governments should put pressure on the Bush administration by pledging to withdraw from Nato if the United States or Israel uses nuclear weapons against Iran. “They should say right now that there will be no Nato if it’s a Nato member that commits a nuclear aggression against Iran,” he said.

“Saying that before the event has a real chance of avoiding that disaster.” Ellsberg shared the 2 million kronor (euro215,000; $273,000) Right Livelihood Award with Indian women’s rights activist Ruth Manoramaand a poetry festival in Medellin, Colombia.

Anti-corruption campaigner Chico Whitaker Ferreira of Brazil won the honorary award. Manorama, who fights for the millions of India’s dalit women, who belong to no caste and have faced centuries of discrimination, said she hoped the award would shed more light on the injustices caused by the Indian caste system. “The world is becoming global but human rights, in our situation, are not global,” she said.