Aintree is gearing up for the biggest steeplechase in the world, the Martell Grand National.

It will be the 155th running of the race and there will be a maximum field of 40 runners on Saturday.

Bookmakers are expecting a massive �50m to be wagered on the race itself and the day's turnover to exceed �120m.

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It is the first National since the introduction of tax-free betting last October.

Bookies estimate that up to 15 million people, a third of the adult population, will have a flutter on the four-and-a-half mile steeplechase.

At Aintree, crowds were gathering and having to undergo strict security screening.

While around the world more than 600 million people are expected to watch the race live on television.

Among the favourites for the big race are two horses who were unlucky in last year's race, which took place in torrential rain.

Blowing Wind is being ridden by record-breaking champion jockey Tony McCoy.

He was passed fit to ride by the doctor on Saturday morning, along with Mick Fitzgerald after they both suffered heavy falls on Friday.

Last year McCoy remounted the horse to come third of four finishers behind the 33-1 shot Red Marauder.

And Beau, the mount of Carl Llewellyn, was travelling really well before the jockey lost his reins in the latter stages.

Other leading contenders include Ad Hoc, runaway winner of the 2001 Whitbread Gold Cup, and Irish raiders Davids Lad and Alexander Banquet.

Trainer Martin Pipe has eight horses in the National, including Blowing Wind, and is looking for his second victory.