The woman so often referred to as the first lady of Australian racing is set to train a racehorse for the Queen of England.

On same the day it was announced the Duchess Of Cambridge was pregnant, Sydney trainer Gai Waterhouse revealed she was waiting on a royal addition to her famous Randwick stables.

Waterhouse has been asked to train Carlton House, the Queen's top-class four-year-old who will race in Hong Kong at the weekend before heading to Australia.

Although Carlton House will be the first horse owned by the Queen to be trained in Australia, her famous royal colours were worn by a runner in the 1997 Melbourne Cup.

Arabian Story finished a creditable sixth in the race won by Might And Power

Waterhouse, holidaying in New York, used Twitter to make her own royal announcement.

"Delighted and grateful to train HM the Queen's Dante and Brig Gerard winner, Carlton House after the colt's HK Vase run this Sunday," she wrote.

The Queen's bloodstock and racing advisor John Warren also confirmed Carlton House would have his last run for leading English trainer Sir Michael Stoute in Sunday's Hong Kong Vase at Sha Tin.

"It is an exciting plan and a wonderful opportunity for Her Majesty to have a horse that can participate in Australia where racing is top class," Warren said in a statement.

"Carlton House looks the perfect candidate to race in Australia where there are a considerable number of options for an older horse."

The Queen's involvement as an owner in thoroughbred racing started in 1952 and she has won some of England's biggest races including the Oaks, the 2000 Guineas, the Coronation Cup and King George VI and Queen Elizabeth Diamond Stakes and the Eclipse Stakes.

Carlton House was beaten into second place by former Australian star So You Think in the Prince Of Wales's Stakes at Royal Ascot this year but Estimate gave the Queen her 21st winner at the famous annual racing festival.

My Kingdom Of Fife, a horse she raced before selling to Australian interests, ironically won the Group One Queen Elizabeth Stakes at Randwick last year - one of three victories under the care of Chris Waller.