As rumours swirl over Justin Timberlake's close night with his female co-star Alisha Wainwright, details about the actor's prenup with Jessica Biel have surfaced.

Several reports reveal that the married stars have a clause in their prenup prohibiting infidelity and if it takes place it would cost the cheater heavily.

Following their marriage in 2012, the New York Daily News reported the couple signed a contract that specified how their property would be divided up in the case of a split.

Timberlake, or Biel, would be financially liable if they were ever unfaithful.

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Timberlake's net worth topped $US200 million ($305m) as of 2019.

"Reports of the Timberlake-Biel prenup infidelity clause in 2012 began a trend in so-called 'lifestyle' clauses [also known as 'love contracts'] within prenups, postnups and cohabitation agreements," asset-protection lawyer Ann-Margaret Carrozza told FOX.

"These clauses can deal with items of concern such as substance abuse, infidelity, weight gain, spending" and more, she said.

Reports haven't specified what would be considered infidelity under Timberlake's contract with Biel, and Carrozza told FOX Business the couple's current residence in Tennessee would also affect the agreement's terms.

Yesterday, Timberlake had spoken out about recent photos showing him in a cosy moment with co-star Alisha Wainwright, describing it as a "strong lapse in judgement".

Last month, the married star was seen sitting on a balcony in New Orleans and holding hands with Wainwright, who at one point was resting her hand on his knee.

In a statement released via Instagram on Thursday, the SexyBack singer apologised to his "amazing wife and family" for putting them through such an "embarrassing situation" – but insisted that nothing else had occurred between himself and Wainwright, news.com.au reports.

"I stay away from gossip as much as I can, but for my family I feel it is important to address recent rumours that are hurting the people I love," Timberlake wrote.

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"A few weeks ago I displayed a strong lapse in judgement – but let me be clear – nothing happened between me and my co-star. I drank way too much that night and I regret my behaviour.

"I should have known better. This is not the example I want to set for my son. I apologise to my amazing wife and family for putting them through such an embarrassing situation, and I am focused on being the best husband and father I can be. This was not that.

"I am incredibly proud to be working on Palmer. Looking forward to continuing to make this movie and excited for people to see it."