Cowgirl Zara goes wild with knee-high boots, a very low top (and a big snog from Mike)

It simply wouldn't do on the racecourse.

And while the theme was still equestrian, there was little else Zara Phillips' daring cowgirl outfit had in common with the demure riding gear she usually sports.

The Queen's granddaughter stepped out at a charity ball in a bold electric blue dress with a neckline plunging to her navel which left little to the imagination.

Yeehaa: Zara Phillips and Mike Tindall at the Red Cross Ball last night

Coupled with a cowgirl hat and boots, it was a world away from the jodhpurs and chaps she wears to compete internationally as a rider.

The Roberto Cavalli dress, which was tied at the midriff with a black ribbon, was cut so low it gave fellow revellers at the Honky Tonk Blues themed event an eyeful of her cleavage and black bra.

If the 26-year-old royal was aware of onlookers gawping at the British Red Cross ball, she did not show it.

Cowgirl kisses: Zara and Mike put on a very public display of affection

Howdy partner: Zara and Mike Tindall get close

Instead she spent the evening cosying up to her rugby-playing boyfriend Mike Tindall, who had also embraced the Wild West theme with a lariat neck tie and cowboy hat.

The sporting couple have always tried to keep a low profile, but last night they were clearly in the mood for fun - at one point the 26-year-old equestrian lassoed in her man, planting a kiss on his lips in full view of the crowd.

She also made time to mingle with the crowd - which included Nancy Dell'Olio, Cheryl and Ashley Cole and Jade Jagger - stopping to chat with fashion designer Roberto Cavalli.

Unlike her cousins - Princes Harry and William and Princess Beatrice - Zara is rarely in the headlines for the wrong reasons.

Got the blues: Zara turned heads in a daring plunge neck dress

But this is the second time in a week that her racy dress sense has raised eyebrows.

She was the only royal that turned up to her grandparents' wedding anniversary wearing a rather short skirt last week.

Zara was voted 2006 BBC Sports Personality of Year due to her efforts in horse riding.

She has been dating Mike, 29, who plays for Gloucester Rugby and was part of the England World Cup winning squad in 2003, for almost four years.