Emma Watson makes red carpet return in outfit made from old wedding dresses

Watch: Emma Watson arrives at the Earthshot Prize Awards in London.

Emma Watson made a welcome return to the red (or should we say green) carpet at the Earthshot Prize Awards in a quirky, recycled outfit.

The Harry Potter star wowed in a backless white tulle dress, which she wore over black flared trousers.

The 31-year-old actor rocked up to the prestigious event in London on Sunday evening in the white asymmetric number, which was made made up of a number of upcycled wedding dresses from Oxfam.

She teamed the unique ensemble with rocky, chunky boots, which balanced the feminine floral lace neckline, frothy tulle skirt and plunging backless detailing in the look.

Watson kept her accessories simple wearing a number of jewellery styles from the Harris Reed x Missoma collaboration, including Moonlight Drop Earrings (star) and the collaboration's Star Of The Show Signet Ring.

The pieces are handcrafted with recycled metal elements to help reduce the environmental impact, which is of course an important credential for Watson.

Meanwhile her make-up was typically minimal and her on-trend, messy bob swept behind one ear.

The Beauty and the Beast star turned to Harris Reed to put together the unusual outfit, with the fashion designer describing it as an "immense honour" to have curated the look for Watson and thanking his team for their hours of hard work.

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The awards were hosted by the Duke and Duchess of Cambridge, who also donned recycled outfits for the occasion.

The Earthshot Prize Awards marked Watson's first major social event since she attended the Little Women premiere in December 2019.

The theme of last night's awards was clearly a subject close to Watson's heart, with the star regularly using her platform to throw light on sustainable living and fashion.

Watch: 7 reasons Emma Watson is a great role model.

Back in 2015 she joined the Green Carpet Challenge, pledging to only wear designers and labels that fit with her environmentally friendly criteria and claiming that every item she would wear on the red carpet would be sustainable.

The Calvin Klein dress, she wore to the 2016 Met Gala in NYC, was a prime example of this goal.

Made entirely from recycled plastic bottles, it hit headlines globally and highlighted the actor's desire to use her platform to make an ethical statement.

"Being able to repurpose this waste and incorporate it into my gown for the ‪#‎MetGala‬ proves the power that creativity, technology, and fashion can have by working together," she wrote in a Facebook post.

Before that Watson demonstrated a desire to use fashion for good after partnering with ethical brand People Tree in 2009.

And last year, she joined the board of directors of Kering, a luxury brand group, in her capacity as an advocate for sustainable fashion.

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Watson is also the face for the Good On You app, which rates fashion brands on how ethical and sustainable they are.

"I will work for anyone for free if they're prepared to make their clothing fair trade and organic," she told Vogue in 2010.