(Adds EU and U.S. officials, paragraphs 11-17)

By Paul Taylor, European Affairs Editor

BRUSSELS, Sept 6 (Reuters) - Europeans and Americans differ widely over whether to use force if diplomacy fails to contain Iran's nuclear programme, posing a major challenge for future transatlantic relations, a major opinion survey shows.

The annual Transatlantic Trends study by the German Marshall Fund of the United States and four European foundations suggests U.S.-European ties are unlikely to recover until there is a new president in the White House and may not improve much even then.

The poll, conducted in the United States and 12 European countries in June and released on Thursday, showed an overwhelming 77 percent of Europeans disapprove of President George W. Bush's international policies.

While Americans and Europeans largely agreed on the main security threats -- terrorism, energy dependence, climate change and the possibility of Iran acquiring nuclear weapons -- they differ widely on the use of force to solve problems.

The divergence was sharpest over how to prevent Iran acquiring nuclear weapons, with 47 percent of Europeans favouring ruling out military action if diplomacy fails and only 18 percent in favour of threatening the use of force.

By contrast, 47 percent of Americans favour keeping the military option and only 32 percent would rule it out.

Iran insists its programme is purely for civilian energy purposes but the West suspects Tehran is pursuing uranium enrichment in the quest for a bomb.

The study showed Europeans had a general reluctance to use force and while most supported deploying troops for peacekeeping and reconstruction in Afghanistan, two thirds were unwilling to commit their soldiers to combat operations against the Taliban.

TIES SEEN WORSE

Despite the emergence of new leaders in Germany, France and Britain untainted by the Iraq war, half the Americans questioned and one third of the Europeans said ties between Europe and the United States have worsened in the last year.

"Angela Merkel, Nicolas Sarkozy and Gordon Brown may offer a new spirit of cooperation in transatlantic relations in Europe, but this year's Transatlantic Trends suggest they will need to tread carefully," the survey organisers said.

Robert Cooper, director general for foreign affairs at the EU's Council of member states, said the survey showed "a real, not a passing phenomena in transatlantic relations".

"It is out of the trading range; it's something new," he told a discussion of the findings.

However, Cooper said he agreed with a separate opinion survey of EU officials showing a belief that the relationship was "basically sound, but it can be improved".

"There is an opportunity here to bring us closer together," he said, noting shared concerns about future energy supplies.

Colleen Graffy, U.S. deputy assistant secretary of state for European affairs, noted shared concerns about terrorism, immigration and refugee issues, energy and global warming.

"What jumps out at me is that we are really wanting to work together in partnership and that there is this growing perception that we are a common democratic community."

Graffy conceded though that "America bashing is a sport", and it might take a change of U.S. administration, changes in Iraq, or in the attitude of the media to alter that.

The state of public opinion appears to contrast with closer cooperation on a range of foreign policy issues from the Middle East to the Iranian nuclear issue and the crisis in Darfur.

Respondents cited the management of the Iraq war and Bush's personal style as reasons for their perception.

More than a third of Europeans in 12 countries surveyed and 42 percent of Americans expect transatlantic cooperation to improve after Bush's successor is elected next year.

But 46 percent of Europeans and 37 percent of Americans believe relations will stay the same whoever is elected.

(Additional reporting by David Brunnstrom)

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