Bargain versions of the bespoke dress worn by the Duchess of Cambridge were yesterday doing brisk business online. There was such demand for the blue polka dot dress by Mayfair-based designer Jenny Packham that her website crashed after word spread on Twitter.

But the empire-line crepe de Chine garment was a one-off created for the duchess, and will never be available to the public, the boutique said.

There were comparisons with the dress Princess Diana wore when she left the Lindo Wing carrying Prince William in 1982.

A worker at Packham’s boutique said: “It was an entirely bespoke design for the duchess, it’s not available to buy. Our website crashed with the demand for it.”

Off-the-peg sequined evening gowns cost up to £3,000 and Kate is thought to have paid from £1,000 for her creation. Yesterday, online retailer Simply Be was offering a similar butterfly-print blue dress for only £15. Elitzia.com invited shoppers to “steal her style” for £29.99. A company spokeswoman said: “There will a mad rush for the high street to recreate this.”