Kate Upton has refused to participate in Guess' investigation of designer Paul Marciano.

The 25-year-old model recently alleged that Marciano sexually harassed and assaulted her in July 2010, following the first day of a photoshoot for a Guess lingerie campaign, and the brand has subsequently announced plans to launch an internal investigation into the claims.

But Kate has refused to participate, writing on Twitter: ''I just learned that Guess hired Paul Marciano's PERSONAL lawyer to run their internal 'investigation. I'm truly disheartened to learn that Guess is treating this abuse of power and sexual misconduct (to multiple women) in such a manner.

''I shared my story to help create long-term change at the highest levels of the industry and corporate ladder. I also wanted to empower others to share their stories to stand up for change - whether as a victim of or as a witness to this behavior. (sic)''

Kate described the investigation as a ''stunt'' and insisted she'd only participate if an independent party was brought in to look at her accusations.

She continued: ''Hearing how this 'investigation' is being conducted, and also knowing a strong fear of retaliation still exists within Guess' corporate offices, is extremely disappointing and upsetting. The #MeToo movement is not one of smoke and mirrors, or for corporate office PR stunts to impress shareholders - it's a call to action for TRUE change.

''To conduct an investigation like this seems like a stunt, and is a clear indication to me that Guess is not taking these claims seriously, nor are they committed to making changes at the highest levels. Until Guess makes a commitment to conduct an unbiased investigation, I have declined to participate. I look forward to participating if and when Guess retains a truly independent firm to run the entire investigation. (sic)''

Last week, it was confirmed that Marciano had temporarily stood down from his day-to-day responsibilities with the company.

Guess has also insisted that it is ''committed to maintaining a safe work environment, and looks forward to the completion of a thorough investigation of all the facts''.