The eight-year-old Charolais tipped the scales at 3,682lbs (1.8 tons) - a staggering 300lbs more - than he weighed one year ago.
He took part in a charity 'guess the weight' contest, where organisers had to use specialist scales used to weigh lorries to calculate his mass.
Gargantuan: The Field Marshall had to be weighed using specialist scales used for lorries
The Field Marshall has overtaken the previous record holder, his former stablemate The Colonel, who stood 6ft 5ins tall and weighed 3,500lbs, before his death in 2005.
Incredibly, the record-breaking bullock is the equivalent of a late teenager and is still growing.
Owner Arthur Duckett, 80, who bought the white steer four years ago, said: 'He's in very good health and there's no reason why he won't keep on growing.
'He's only eight and unless something unforeseen happens he will get bigger and bigger. But he's not fat - he's all muscle.
'I could feed him heavier but I don't want to make him look grotesque, I want him to be healthy and put on weight naturally.
'That's why I keep him outdoors in a field and not inside.'
The Field Marshall shares a field on Mr Duckett's farm in Alstone, Somerset, with a 5ft 6in Highland steer and a small Friesian.
He is now heavier than a Mini Cooper car which weighs 2,458lbs, and a BMW 3 series which tips the scales at just 3,053lbs.
Arthur, just 5ft 8ins tall, puts The Field Marshall's incredible size down to his parentage, appetite and the fact that he has been neutered, which boosts his growth.
He eats more than 17lbs of feed every day and enjoys oats, barley, potatoes, fodder beet followed by a portion of hay.
Healthy: Owner Arthur Duckett says there is no reason why the giant bullock won't keep growing
The Field Marshall was weighed at the Fatstock agricultural show at Sedgemoor Auction Centre near North Petherton, Somerset.
Visitors paid £2 to guess his weight and the winner received a tonne of cattle feed or the cash equivalent.
The charity event raised £1,641 for Taunton's Musgrove Park Hospital in Somerset.
Arthur said: 'I haven't force-fed him he just has the same food every day. Oats, barley, potatoes, fodder beet and also a portion of hay.
'He's got a very good temperament but he's not really domesticated so you have to treat him with respect.
'He's very strong and he's got a mind of his own but we get along very well.'
Three years ago Arthur and his wife Helen broke records with their 6ft 5in bull The Colonel.
The Simmental Holstein breed had to be put down in August 2005 at the age of nine because of back problems.
There is no longer an official record for the world's biggest bullock as Guinness World Records does not want to encourage overfeeding.
The previous holder for Britain was a bull in Essex that died in 1830 weighing 4,480lbs.
The last record to be recognised was set by a steer called Old Ben, who died in Kokomo, Indiana, in 1910. He weighed 4,720lbs and stood 6ft 4ins tall.