LONDON — Prime Minister Brown of Britain announced plans yesterday for new European sanctions against Iranian banking, oil, and natural gas interests, signaling a growing willingness by Western allies to join President Bush in punishing Tehran for its nuclear enrichment program.

With Bush by his side at 10 Downing Street, Mr. Brown said Britain and the other members of the European Union had agreed to freeze the assets of Bank Melli, Iran’s largest bank, and would consider separate sanctions targeting Iran’s oil and natural gas industry. E.U. ministers are to take formal action as early as today, officials said.

Mr. Brown’s announcement, which followed meetings with Mr. Bush, was aimed at ratcheting up pressure on the Islamic republic to curtail its nuclear activities and allow more-extensive international inspections.

“We will take any necessary actions so that Iran is aware of the choice it has to make: to start to play its part as a full and respected member of the international community or face further isolation,” Mr. Brown said.

The endorsement of sanctions was a notable victory for Mr. Bush, who is entering his final months in office and, like Mr. Brown, is struggling against low approval ratings and sharp political opposition at home. Mr. Bush made Iran’s uranium enrichment program a key focus of his week-long trip to Europe, which ended yesterday here in Britain.

At his first stop last week, the Balkan republic Slovenia, Mr. Bush and E.U. leaders issued a joint statement threatening the kind of financial sanctions announced yesterdays. Over the weekend, America and five European nations offered a package of incentives to Iran in exchange for a halt to its nuclear efforts.

Iran says its nuclear program is peaceful and has declared repeatedly that it will never give up its uranium enrichment program. But Mr. Bush and other Western leaders fear that the enriched uranium could be diverted to nuclear weapons.

European action against Bank Melli would come on top of restrictions imposed by the American Treasury Department against Melli and other state-owned Iranian financial institutions this year. Between 2002 and 2006, Bank Melli sent $100 million to Hamas, Palestinian Islamic Jihad, and other organizations that America has designated as terrorist groups, according to Treasury officials.

Treasury has also moved to cut off another major Iranian bank, Bank Saderat, from the international financial system.

America has had numerous other measures in force against Iran since shortly after the seizure of the American Embassy in Tehran in 1979.

Mr. Bush’s national security adviser, Stephen Hadley, told reporters on Air Force One that the sanctions show close agreement among America and its European allies on Iran. “I think there was a lot of questions some of you had about whether we were knit up with the Europeans on Iran policy before we left,” Mr. Hadley said. “I think it’s pretty clear that the answer is yes.”