The 2017 Grand National winner One For Arthur will miss his prep race for this year's Aintree contest because of new rules introduced as a result of the equine flu outbreak.
Trainer Lucinda Russell told BBC Sport the horse had to be vaccinated and will not run at Haydock on Saturday.
"It's disappointing but hopefully we can find another race for him," she said.
Bristol De Mai heads the weights for the £1m race at Aintree on 6 April.
The eight-year-old grey, trained by two-time National winner Nigel Twiston-Davies, has been allocated 11st 10lb.
Last year's victor Tiger Roll, seeking to become the first dual victor since Red Rum in the 1970s, will carry 11st 1lb if he runs.
A maximum of 40 horses will line up for the race, with Sandymount Duke - owned by Rolling Stones guitarist Ronnie Wood - the last horse guaranteed a run.
The National is a handicap chase where each horse is given a weight according to their rating.
A total of 110 entries remain at this stage, with lower-rated horses eliminated nearer the time.
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British horse racing will resume on Wednesday after a six-day shutdown because of the equine influenza outbreak.
A total of 174 racing stables were placed in lockdown after six horses tested positive for the highly contagious virus at the Cheshire stables of Donald McCain.
Four further cases were discovered at Newmarket trainer Simon Crisford's yard.
The British Horseracing Authority gave the all-clear late on Monday night for racing to resume on Wednesday.
It has introduced new rules which require horses to have been vaccinated within the last six months, rather than a year.
"He seems in really good form. We'd like to run him before the National," said Russell, who trains One For Arthur for owners who are registered as 'The Golf Widows'.
One For Arthur (given 10st 10lb this year) was only the second Scottish-trained trained winner of the famous race, which takes place over 30 fences, including Becher's Brook and The Chair.