U2 frontman Bono has promised that the band's next studio album will be their “best ever”.

The as-yet-untitled project had been due later this year but Bono said it has been delayed until Spring because “there's more priceless stuff to be found”.

“We know we have to emerge soon but we also know that people don't want another U2 album unless it is our best ever album,” he wrote on the band's website.

“It has to be our most innovative, our most challenging ... or what's the point?"

U2 are currently working on the record in the South of France having previously recorded tracks in Dublin with producers Brian Eno and Daniel Lanois.

Bono revealed that “50 or 60” songs were in consideration for the album, which is the follow up to 2004's 'How To Dismantle An Atom Bomb'.

"The last two records were very personal, with a kind of three piece at their heart, the primary colors of rock -- bass, guitars and drum," he wrote.

"But what we're about now is of the same order as the transition that took us from 'The Joshua Tree' to 'Achtung Baby.'"

As previously reported, four songs purported to be from the album were posted on YouTube recently after a fan recorded the singer playing them to friends in France.

(by Jason Gregory)