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The Prince of Wales and Duchess of Cornwall have attended the opening day of the Hay Literary Festival in Powys.

The royals met traders and residents as the town hosted its annual gathering of authors, politicians and celebrities.

Washington journalist Carl Bernstein, spy novelist John le Carre and actress Miranda Hart are among the attractions.

The royal couple were more than two hours late as they travelled by road after a technical fault forced their helicopter to land just after takeoff.

Just minutes after leaving central London, an in-flight emergency forced the helicopter to land at Denham aerodrome in Buckinghamshire.

"The pilot carried out a controlled emergency landing after diverting to the airport," a spokesman for the royal couple said.

The royal party then continued their journey to Hay-on-Wye by road.

The late arrival meant the tour had to be shortened, but the royal couple were still able to visit regeneration projects in the town and meet local shopkeepers and residents before heading to the festival.

This year's event, which runs until 2 June, is the 26th to be staged in the border town, famous for its second-hand book shops.

It will mark the Iraq war's 10th anniversary with a lecture from Swedish politician and diplomat Hans Blix, the UN inspector who led the search for weapons of mass destruction in Iraq between 2000 and 2003.

The festival will also celebrate the centenary of the birth of RS Thomas, the Cardiff-born poet who grew up on Anglesey.

In the past, the festival has attracted former US presidents Bill Clinton and Jimmy Carter, as well as former vice-president Al Gore.

This year award-winning American journalist Carl Bernstein, who broke the Watergate scandal in the Washington Post in 1972, will reflect on the Barack Obama administration.

He will also discuss the American anniversaries of Gettysburg (150th), the Vietnam war (40th) and President John F Kennedy's assassination (50th).

Nobel Peace Prize winner Jody Williams, who campaigned to ban landmines, and former Beirut hostage John McCarthy will also speak at the event, along with the former Archbishop of Canterbury Dr Rowan Williams.

John le Carre, the creator of George Smiley, will make his debut at the festival, while other authors include Sebastian Faulks, Lionel Shriver, Caitlin Moran, Irvine Welsh, Howard Jacobson, Thomas Keneally, William McIlvanney and Alexander McCall Smith.

Call the Midwife star Miranda Hart will talk about her work in comedy and drama, with fellow actors Robert Lindsay, Rupert Everett and Hugh Dennis also making festival appearances.

Hay Festival director Peter Florence said the festival site had been redesigned and "Tardised into a bigger, more open space".

"It's wonderful, and it looks fabulous in the sunshine," he said.

"We also have a thrilling programme of music from Mali featuring Rokia Traore, Terakaft and Amadou & Mariam to celebrate Hay's twin town of Timbuktu."

Welsh culture and sport minister John Griffiths said the arts were more important than ever.

"No-one will deny the fantastic contribution our creative sector makes to the nation's economy, but as Hay exemplifies every year the arts mean so much more to us as a country than pounds spent.

"Indeed, people want, perhaps even need the arts more in difficult times."

Later, Prince Charles attended the Welsh National Opera's opening night of Lohengrin at the Wales Millennium Centre in Cardiff.