The millinery department of John Lewis is reporting a 20% increase in sales of hats and fascinators, credited to the much-lauded “Kate Middleton effect” as royal wedding fever builds. The trend is catching on in the US too, with many Americans reported to be buying hats to attend one-off royal wedding viewing parties. Google searches for fascinators – a favoured style of Miss Middleton at formal events – have jumped 50% in the US this year.

Vernell Washington, owner of Grand Diva Enterprises and the go-to hat lady in Atlanta, Georgia, has seen a boom in orders across the US in recent weeks. She said: “It’s been astonishing. The phone has been ringing off the hook.

She credits huge interest in the royal wedding for the sales. “Thanks to Kate, it is exploding. Hats are the hottest trend right now. In recent years a lot of women have not been wearing hats because they considered them stuffy or old-fashioned but Kate has put a younger twist on it.”

Clients are buying hats, she said, to wear at royal wedding-themed events. She added: “There are a lot of people in Atlanta throwing royal wedding parties. They are agreeing not to watch the wedding live, but instead taping it in the morning then meeting in the evening to drink champagne and watch it together.

“One group of ladies have all bought fascinators from me and will stay up late to watch the wedding in their pyjamas and fascinators. “I have other clients who will be wearing their hats to special tea parties and royal wedding-themed events later in the day. It really is reaching every gamut.”