The annual fusion of high-end fashion and top-class racing which is Royal Ascot comes with added spice this year, thanks to the presence of two outstanding fillies.

Estimate, owned by Queen Elizabeth II, attempts to win the meeting's showcase, the Ascot Gold Cup, for the second year running on Thursday.

And Treve bids to advance her status as the world's best racehorse in the Prince Of Wales's Stakes on Wednesday.

They are the headline acts during a week when total prize money has risen to STG5.31 million ($A9.69 million), a six per cent increase on last year.

The running order of races has been amended this year. The feature race will now be run as the fourth race each day as a result of extended live terrestrial television coverage of all 30 races.

Qipco, a private investment company for members of the Qatari ruling family, has been embraced as an official partner for the first time.

The arrangement represents a relaxing of Royal Ascot's previously rigid stance on commercial backing and on-site branding.

Tuesday's opening day features three championship races, when runners from France, Ireland and South Africa oppose a strong British-trained contingent in the Queen Anne Stakes.

Unusually, however, there are no runners from Australia or Hong Kong in Tuesday's King's Stand Stakes or the Diamond Jubilee Stakes on Saturday.

Horses from those countries have posted six victories in the two championship sprints since Choisir, trained in Australia by Paul Perry, won both races in 2003.

In their absence the international baton passes to Mike de Kock and Wesley Ward.

De Kock, from South Africa, saddles Soft Falling Rain in the Queen Anne and Shea Shea, last year's runner-up, in the King's Stand.

Horses trained by Ward in the US will contest many of the meeting's races for two-year-olds, starting with Hootenanny, ridden by Victor Espinoza, in the Windsor Castle Stakes on Tuesday.

But all eyes will focus on Estimate when the Queen's horse lines up in the Ascot Gold Cup. The five-year-old's preparation has been complicated by a muscle injury.

Although Estimate has not run for eight months, Sir Michael Stoute, the leading active trainer at Royal Ascot with 68 winners, is confident she is ready to give her best.

Estimate bids to win at Royal Ascot for the third year running, having won the Queen's Vase in 2012.

The Queen is scheduled to attend on each of the five days.