Duke of Marmalade, Henrythenavigator and Yeats are preying on the minds of William Hill's oddsmakers as Britain's second-largest bookie reported a 10% slump in profits, and another caning by punters in the past week.

Poor horseracing results, notably at Royal Ascot, was put up as the main feature of a poor first half for Hills. In an extraordinary run in the past few months, 16 Group one races, have gone to Irish trainer Aidan o'Brien, many of the winners ridden by his jockey Johnny Murtagh and backed by the firepower of bloodstock tycoon John Magnier and his former bookmaker co-investors Michael Tabor and Derrick Smith.

"Yes it has been the Aidan effect," said chief executive Ralph Topping. "We are just happy he doesn't coach football teams as well.

"His is a quality stable, he's on a great run and he will be known as one of the all-time greats. There is no point in trying to work out how much he has cost us." punters lumped on o'Brien's Duke of Marmalade in the King George at Ascot on Saturday and Henrythenavigator yesterday - and cleaned up.

Racing makes up about 50% of Hills' business and football a fast-growing 15%. Euro 2008 was not much better for Hills either as punters saw Spain's fantastic start to the tournament and backed them heavily to victory three weeks later.

The worst effects were felt in Hills' telephone betting business, where bets are on average eight times as big as at the betting shop. Hills' wins from phone bets slumped 22% and profits more than halved to £4.5 million. The City has marked Hills' shares down 50% in the past nine months on fears it is a sitting duck in an economic downturn.

Topping countered that since the last recession in the early 1990s, Hills' business has been transformed by gaming machines and video casino games in betting shops, and by the advent of internet betting.