For centuries, the Ascot race meeting has been one of Britain's premier social occasions, patronised by royalty and attended by the cream of society in all their finery.

But upper-class eyebrows have been raised in recent years by what many race-goers perceive as a distinct lowering of the tone when it comes to the standard of dress.

Short, strapless dresses, jeans and even shorts have increasingly been spotted amongst the top hats and tails -- and now the organisers are fighting back.

They have issued new guidelines about what is and what is not acceptable for ladies and gentlemen to wear to Ascot.

Sheer strap tops are banned from the 300-year-old event - which is attended each day by members of the royal family - midriffs must be covered and trousers for women and men must be full length.

Sportswear, jeans and shorts are strictly forbidden.

Ladies visiting the Royal Enclosure are advised to wear dresses or skirts of a modest length that fall just above the knee or longer.

Hats are allowed or a headpiece which has a base of 4 inches (10 cm) or more in diameter, is deemed as an acceptable alternative to a hat. That means that fascinators, a firm favourite of the queen's granddaughter-in-law the Duchess of Cambridge, are no longer permitted in the Royal Enclosure.

Gentlemen are required to wear either a black or grey morning suit, which must include a waistcoat and tie (no cravats) with a black or grey top hat and black shoes.

We have worked extensively with experts in the world of fashion to define better what formal dress means, Ascot Chief Executive Charles Barnett said in a press statement.

It isn't a question of elitism and not being modern in a world where there is less and less requirement to dress smartly - far from it.

A morning suit can be purchased from British retailer Marks and Spencer for as little as 99 pounds, not far off the price of a branded sportswear top combined with trousers.

We want to see modern and stylish dress at Royal Ascot, just within the parameters of formal wear, and the feedback we have received from our customers overwhelmingly supports that.

Stewards will start to enforce the no leisure wear rule in the coming months.

The Royal Meeting will be held from the 19th to 23rd June this year.

(Reporting by Li-mei Hoang, editing by Paul Casciato)