Nine-year-old Oscar with the 14cm by 10cm giant flake found by his father Steven
Steven Marriott, 44, wondered why nothing was coming out when he opened the new box for nine-year-old Oscar.
But he quickly realised a massive single bran flake was blocking the inner bag of the Kellog's cereal.
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The giant flake, which measures 14cm by 10cm (5.5in by 4in), eventually fell out of the box along with a small piece measuring 7cm by 4cm (2.7in by 1.5in).
The pair discovered the huge flake as they made breakfast on Wednesday morning.
Mr Marriott, who lives in West Bridgford, Nottinghamshire, said: "I opened a new box and tipped it up and nothing was pouring out because of this massive bran flake.
Steven found the monster size flake after it had prevented the other flakes coming out of the box
"It's definitely bran flake material but I didn't fancy eating it.
"The whole family were around getting ready for work and school so I told them to come and have a look.
"A bit had even broken off in the box so it would have been bigger. It still exists, I've not demolished it.
"It is being kept in the cupboard at the moment but I am not sure what we are going to do with it."
Oscar's father sent a photo of the giant flake to Kellogg's in the hope of receiving a free box
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Mr Marriott, who bought the £2.89 box of Bran Flakes from Waitrose, said he posted a picture of the giant flake on Twitter.
The IT manager, who lives with his wife Clare, 43, and their three children Ethan, 15, Elana, 13, and Oscar, added: "I thought it was interesting so I sent Kellog's a picture of it to show them.
"I wasn't moaning but I thought they might send us a free box or something. I haven't heard back off them yet though.
Kellogg's have congratulated Mr Marriott for finding the super bran flake
"We've still got the flake and considering putting it on eBay. Apparently an over-sized cornflake was sold for more than £1,000 a few years ago so you never know."
Kellogg's have now congratulated Mr Marriott and Oscar for finding the super-sized flake but jokingly apologised if it "dominated" his bowl.
A spokeswoman for the cereal giant said: "We would like to congratulate Mr Marriott on finding a super Bran Flake.
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"We are unaware of any other Kellogg's Bran Flake being created to this enormous scale and would love to know if it is the largest in the world.
"We apologise if the giant flake dominated his cereal bowl.
"At Kellogg's we like to encourage all of our customers to have fun with their cereal and come up with new, versatile recipes.
"Perhaps there is a fun and alternative way to enjoy this extra-large bite."
In 2008 a giant cornflake in the shape of the US-state Illinois fetched £1,000 when it was sold on eBay.
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