She is believed to be 'telling all' about her romp with Liam Gallagher at 35,000 feet in an explosive new memoir.
But Lily Allen, 32, previously denied sleeping with Nicole Appleton's then-husband in a phone conversation to the All Saints singer, 43, a new report suggests.
The Smile singer is believed to have insisted 'nothing went on' with Liam, 45, on a boozy 11-hour Virgin Atlantic flight to Japan in 2009, even though the details are expected to follow next year.
A friend told The Sun on Monday: 'Nicole is gobsmacked about the whole thing. At the time, Lily even called her up to insist nothing went on between her and Liam.
MailOnline has contacted representatives for Lily Allen and Nicole Appleton.
Claims: Liam, 45, was just a year into his marriage with All Saints singer Nicole he allegedly romped with Lily in the First Class section of their flight to Japan in 2009 (Liam, Nicole and Lily are pictured in 2007)
Set to be revealed in her new book My Thoughts Exactly next year, Lily is believed to have had a booze-fuelled tryst with the former Oasis star on the way to Fuji Rocks Festival in 2009, The Sun reported this weekend.
Liam was just a year into his marriage with All Saints singer Nicole when they allegedly romped in the First Class section.
In response to the 'tell-all,' Nicole issued a stark warning to Lily on Sunday morning.
Yet Nicole's Twitter followers saw the ironic side of her latest tweet, which comes just weeks after she was swept up in the brief marital split between Paddy McGuinness and his wife Christine.
Last month, Nicole was pictured arm-in-arm with Paddy on a boozy night out in London while his wife was at home looking after their three children.
While neither party ever confirmed the details of their evening out, it led to a brief split between Paddy and Christine, who patched things up later in the month.
'I’d ask Christine McGuinness about that,' suggested another fan on Twitter.
Allegations of a tryst between Liam and Lily come four-years after Nicole's marriage to the Paper Crown singer collapsed following his fling with American journalist Liza Ghorbani, who later gave birth to his daughter, Gemma.
'Lily's agent has been touting the first draft of the book and boasting that nothing has been held back,' an insider told The Sun.
The book is also said to revisit an awkward encounter when Lily was said to have ran into Liam and his then-wife Nicole.
'It was fairly soon after the flight, which had sparked gossip in pop circles. Lily made a beeline for Liam and Nicole and started chatting.
'But Nicole was not happy about it, as she had heard the gossip and it caused a few ructions between her and Liam,' the source added.
The former Oasis singer married songstress Nicole Appleton in a romantic ceremony in February 2008, just 17 months before the alleged fling.
His first marriage to actress Patsy Kensit, with whom he shares 18-year old son Lennon, ended 2000, three-years after they exchanged vows.
While the musician sees his sons regularly, he previously confessed that he is yet to meet children Gemma and Molly, his 19-year old daughter with singer Lisa Moorish.
MailOnline has reached out to Lily Allen and Liam Gallagher's representatives for further comment.