Designer Matthew Williamson for the first time opened up about dressing Kate Middleton. The 44-year-old British designer, who has a new collection with Debenhams, spoke to Hello about dressing the royal, the effect her appearance had on his eponymous brand, and the "Kate effect."

The fashionable royal recycled her gray Matthew Williamson dress when she arrived for an official engagement at the Place2Be Headteacher conference on Nov. 18. She looked chic in the Matthew Williamson embellished wool crepe peplum dress. The short dress featured high neck, short sleeves, contrasting green embellishment at the neck and cuffs and a figure flattering peplum waist.

She made a rare public speech at the conference, talking about the topic close to her heart – mental health. Kate accessorized with her diamond and sapphire engagement ring, diamond drop earrings, and a metal watch. The mother-of-two also carried a navy suede clutch bag and wore matching high heels. Kate had last worn the sold-out dress back in 2012.

For Matthew, when Kate stepped out wearing his design, it was a big moment for him and the effect was “phenomenal.” “The dress she was wearing sold out straight away. We just couldn’t make enough, we sold out of everything we had!” he said. The designer said he was aware that Kate might opt to wear one of his designs and ahead of her visit to Place2Be charity he altered the dress.

Matthew said that Kate wearing his design gave a huge boost to his brand globally. And, since she is a global style icon, his name got known worldwide. The designer said he does not know of anyone who had such a huge effect on fashion.

Matthew has been designing collections for Debenhams since 2001. His new collection brings together his signature glamour to the high street. He said he loved the way his brand could reach a bigger audience via Debenhams.

His designs are for women of all age groups and his 68-year-old mother refuses to wear anything but Matthew Williamson. Matthew said his brand is all about “celebrating women and optimism and having fun.” According to him, apart from Kate, reality TV star Kim Kardashian West has a huge influence in the fashion world.