The Pussycat Dolls singer has been dating the British Formula One ace for nearly three years, but they have reportedly not seen each other in three weeks and shelved plans to spend Christmas together.
A source told the Daily Mirror newspaper: "Lewis and Nicole have had a few problems for a while now. They love each other dearly but have incredibly busy schedules.
"Nicole was in London last week and was desperate to hook up with Lewis but she was busy in the studio while he is on a European training camp and couldn't come back.
Lewis and Nicole have had a few problems for a while now. They love each other dearly but have incredibly busy schedules.
"Hopefully after a month apart, they will come to their senses and give things another go."
Despite the claims, a spokesperson for Lewis insisted the couple are "perfectly fine".
Meanwhile, Nicole recorded the theme song to forthcoming musical 'Love Never Dies' - the sequel to the hugely popular 'Phantom of the Opera', by composer Lord Andrew Lloyd Webber - while in London.
A source said: "Andrew is a huge fan of Nicole and her voice. He has always said she could become one of the biggest stars on the planet with the right songs."