JERUSALEM, May 15 (Reuters) - U.S. President George W. Bush will tell Israel's parliament on Thursday that letting Iran acquire nuclear weapons would be an "unforgivable betrayal of future generations".

"America stands with you in firmly opposing Iran's nuclear weapons ambitions," Bush will tell Israeli legislators on the second day of his visit to the Jewish state, according to an advance copy of a speech he was due to deliver later in the day.

Bush was in Israel to celebrate the Jewish state's 60th anniversary and try to shore up the faltering Israeli-Palestinian peace process.

In his speech to the Knesset, he planned to hammer home his view that democracy could prevail against extremism in the Middle East, where he has struggled to push his "freedom agenda".

His strongest criticism will be aimed at Iran, Israel's main foe in the region.

"Permitting the world's leading sponsor of terror to possess the world's deadliest weapon would be an unforgivable betrayal of future generations. For the sake of peace, the world must not allow Iran to have a nuclear weapon," Bush will say.

Bush has led an international campaign to isolate Tehran diplomatically over its nuclear ambitions. He says there is a danger that Iran will use nuclear technology to build a bomb. Tehran says its programme is for peaceful civilian purposes.

Bush has accused Iran of fomenting violence against Israel by Palestinian militants, of using Hezbollah to try to destabilise Lebanon's elected government and of arming and training Shi'ite militias in Iraq. (Reporting by Matt Spetalnick; editing by Andrew Dobbie)

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