Button up your raincoats, it’s going to be a stormy season for luxury.

Burberry — whose tartan-emblazoned trench coats and accessories have long filled the closets of the wealthy — warned yesterday of surprisingly sluggish sales for the remainder of the year due to a worsening global economy.

The frightening forecast sent shares of the London-based luxury group plunging 21 percent, their steepest-ever one-day drop, which erased more than $2 billion in market value. The company admitted its comparable sales have been flat during the past 10 weeks.

In addition to being hurt by lackluster economics, luxury goods, including name-brand fashions and accessories, are seeing lower sales because they are losing their cachet with the upper crust, says Pam Danziger of Unity Marketing, a luxury research firm.

“People don’t want Rolexes anymore,” Danziger said. “The new status watch is a triathlon watch — it shows you accomplished something other than being rich.”

Shares of luxury companies in the US including Ralph Lauren, Tiffany and Coach dipped on the news.