In a statement Rebecca Loos, Beckham's former personal assistant, said: "Since the News of the World broke its story, I've read an awful lot about myself much of which is either untrue or grossly exaggerated.
"So I will now tell the true story of my affair with David Beckham to the News of the World in my words."
Loos, 26, is reported to have signed a £300,000 (€450,000) deal to reveal all about the alleged affair, which Beckham, 28, denies.
A spokesman for the Beckhams said they would not comment. He is expected to play for Real Madrid tomorrow, with wife Victoria expected to fly to Spain to watch the game in a public show of unity.
Beckham missed Wednesday's Champions League quarter-final defeat against Monaco due to a one-match suspension.
The former Manchester United midfielder reported for training in the Spanish capital on Thursday, leaving Victoria on the ski slopes of French resort Courchevel with sons Brooklyn and Romeo. Dressed in a black woolly hat, he appeared relaxed as he chatted with team-mates and showed no sign of worry about the impending revelations.
Meanwhile, Victoria insisted the couple's marriage was "fine" and in an as yet unbroadcast interview the former Spice Girl reveals that she and David are "practising" making babies.
She made her confession to pint-sized presenters Little Ant and Dec on ITV1's Ant & Dec's Saturday Night Takeaway. In the interview, which was recorded two weeks ago, the cheeky duo asked her: "Will you have any more babies?"
"I definitely want to have more babies," she replied.
When asked: "Are you and David making more babies at the moment?" she told them: "We're not planning on making babies at the moment. We're what they call practising at the moment, which is really good fun."
The interview will be shown on Saturday night.