The Duchess of Cambridge may turn heads with her classic style but sister Pippa is now rivalling Kate's timeless fashion thanks to a little-known British designer.

When Pippa and the Middleton family joined the Queen, and William and Kate, at a Sandringham church service a few weeks ago, she was not wearing a label costing thousands of pounds.

Instead she opted for the simple elegance of a bespoke coat dress from the family-run firm Browne & Daughters, which has been dressing the country set, and townies, for more than five years.

And like the "Kate effect" which can see dresses worn by the Duchess sell out in hours, since Pippa showed an interest in the outfit called the Fiona Dress, named after its creator, it has proved hugely popular.

Designer Fiona Browne started the business in 2009, fulfilling a long-held ambition to see her fashion creations come to life after her family had grown up and left the nest.

She has been joined in the business by her three daughters - Lucinda, Georgie and Clemmie - who look after the essential backroom roles from marketing to accountancy.

Fiona was discreet about Browne & Daughters' relationship with their most famous client, but she praised Kate's little sister for choosing the right moment to step out in her bottle green tweed coat dress, with its velvet trim and cuffs.

"She managed to find an opportunity to wear the Fiona Dress, and she could not have looked more fitting then at that church service," said the designer who is in her 60s.

The outfit's price starts from £420, with the client receiving a bespoke service to tailor the garment to their individual style.

Mrs Browne added that Pippa, 32, may have told friends about her fashion find, soon after she bought it last year, as it is proving a best seller.

She said: "The Fiona Dress has been popular, I doubt if it's a complete coincidence it's flown out the door. Since Christmas there's been a huge interest in it - I guess it's the Middleton touch."

Pippa's choice caught royal fashion watchers by surprise as all failed to identify the designer.

Recent highlights from Pippa's wardrobe have included a stylish cape by Tania Cashmere, a kaftan by Biondi for a sunshine break and smart pieces like the French Connection wool coat she chose when she joined her mother Carole at a memorial service attended by William.

Clemmie Browne, who looks after client fittings from the company's office in west London, said her mother created "timeless and elegant pieces that will last" adding: "Her inspirations are figures like Audrey Hepburn and other icons from the 1960s, but she puts her own stamp on it."

Browne & Daughters is based close to Lambourn village, which is near the Middleton family home in Bucklebury, Berkshire.

And there is another connection between the label and Kate, as Clemmie's close friend Catherine Zoraida is a jewellery designer whose pieces have been worn by the Duchess.

Fiona said fathers had classic pieces of tailoring to hand down to their sons like a cashmere overcoat, but that rarely happened between mother and daughter.

She added: "We don't feel bespoke needs to be for the uber rich, it's also for anyone who just want to spend a good amount on clothes that are stylish and stand the test of time.

"The piece that Pippa wore, it's always going to be elegant and beautiful."