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"It felt like the right time to do this," says Paramore's Hayley Williams with a deep breath. A long pause follows. "It's our first ballad."

New single The Only Exception might come to casual fans as something of a shock. It's not the usual skid-wheeled glossy punk the Tennessee fivesome produce.

"It's our first love song that speaks about love in not a negative way. It's a more positive light," she says.

"It's about me learning to believe that love can happen and exist in this world where I've never experienced it or witnessed it before. I'm really stoked about that message."

It is easy to forget that Hayley Williams is only 21 and that the million-selling band she fronts were signed when she was 16.

"It was written with someone in mind," she continues. "I feel really lucky to have felt what I considered to be something authentic and genuine and pure. I'm still pretty young.

"I wouldn't say that getting older has anything to do with playing a slower song.

"This song is definitely different for us and it's a side we've never shown but I think the typical Paramore sound that everyone is used to, that is who we are."

A change of direction indeed, and the slowest track of most recent album Brand New Eyes released last year.

The video features Williams lying on a bed of cards sent from fans of the band.

"Over the Christmas holiday we got so many fan mails and Christmas gifts - crazy things from fans," she explains.

"It was really amazing to see how much time and effort people want to give to you that's a piece of them.

"When we put the email blast about it we had 400 cards in a week. They were all really personal."

As well as playing packed arenas in the UK last autumn (Wembley sold out in four hours) they toured the US alongside reformed ska-poppers No Doubt, fronted by Gwen Stefani.

"It was a dream come true," enthuses Williams. "I still can't believe we did it.

"When I'm 40 years old I want to be doing what Gwen's doing. It was really motivating."

The single isn't the last we'll be hearing from them, they're playing this year's Reading and Leeds festivals as well after two stateside jaunts. They're prepared to keep going.

"I get bored of maybe five or six days at home," she laughs.

"I'm like, 'Get me out of here, I'm don't know what to do with myself'."

The Only Exception is released on Monday 5 April