Lily Allen has detailed her sex romp with Liam Gallagher.

Lily Allen

Lily Allen

The 33-year-old singer met the former Oasis frontman on a plane on the way to Japan for the Fuji Rock Festival in 2009 and the pair proceeded to get drunk and have sex in the toilets, before heading to his hotel together after they landed.

In an excerpt from her upcoming autobiography, 'My Thoughts Exactly', obtained by The Sun on Sunday, Lily writes: "We got hammered on the plane. At some point Liam and I found ourselves together in the toilet doing something that he shouldn't have been doing, and it wasn't drugs. Then we were in a lie-down bed together.

"When we landed in Tokyo, Liam said, in his distinctive nasal twang, 'What you doing now? Why don't you come back to our hotel' . . .  I was like, 'Yeah, OK'."

However, Liam, 45, was married to All Saints singer Nicole Appleton at the time and he begged Lily never to reveal their fling.

Lily adds: "It wasn't until the next day when he said something like, 'No one can hear about this because of Nic,' that I clocked he was married. I knew Noel was married and I knew Liam had been married to Patsy Kensit. That was partly why I fancied him. I'd grown up an Oasis fan. I was 12 when he and Patsy Kensit were on the cover of Vanity Fair as the coolest couple."

However, six months later Nicole discovered the truth and Liam asked Lily to lie to his wife.

She writes: "Six months later I was in my car with my driver when the phone rang. . .  It was Liam. He sounded awful, like he'd been up all night. 'Lil,' he said. 'I just need you to get on the phone. I've got Nic here,. One of your mates is, like, trying to do a wind-up or something and saying that you and me had it off or whatever, and I need you to speak to Nic and say that it's all b*****ks.'

"Oh, Liam, yes sure, it's all b*****ks. Let's say it's all b*****ks -- of course I'll tell Nicole that it's all b*****ks if that's what you want -- and not think about it too much or even at all, and then maybe it will go away and not disrupt what really matters: The people we love and rely on."