The rest of the band, Mark, Ben and Christian, said they are shocked but they'll try to carry on without him.
Paul said: "After a lot of thought and deliberation I have made the
difficult choice to leave the band due to personal reasons.
"I have had a brilliant five years with A1 and would like to take this opportunity to
thank everyone who has been part of the whole A1 experience, everyone at
Sony, but most of all our amazing fans.
"I would like to wish Christian, Ben
and Mark all the best of luck in the future."