The drawings, by renowned artist Laurence Broderick, were discovered almost two years after his death aged 88.
They were stashed under a bed at his family’s home in Cambridgeshire.
Broderick was an internationally acclaimed sculptor specialising in figurative carvings in stone and castings in bronze, often with a nature theme.
He was best known for his six-ton, 4.5m long bronze bull sculpture - named one of the world's top ten public artworks.
The Bull was erected in 2003 at the Bullring shopping centre in the centre of Birmingham.
The portfolio of drawings is expected to sell between £2,000 and £4,000 but could fetch far more when they go under the hammer.
More than 50 lots are being sold at Batemans Auctioneers in Stamford which is expected to attract bidders from around the world.
Auctioneer Stephen Drake stumbled across the unseen sketches while assessing the value of the family’s estate following the death of Broderick’s wife Ingrid last August.
He said: “I was visiting the family home to assess other related works.
“We knew there would be many interesting items and really hoped we would find something relating to the Birmingham Bull.
“It was a fantastic moment when the incredible portfolio of his work was pulled from under the bed.
"I have done the job of an auctioneer and valuer for over 27 years and it’s moments like this that make my job the best one in the world.”
In the portfolio are the original sketches for the bull, complete with handwritten notes by Broderick about his favoured design.
The collection, comprising 14 signed A2 sketches and dated July 2001, were scribbled two years before the The Bull was unveiled.
On the final sheet, Broderick wrote: "14th drawing and it was the drawing I decided to make."
The bronze, officially titled The Guardian, was erected in 2003, as the focal point of the city's shiny new Bullring shopping centre.
At the time, Broderick described it as a "gateway emblem for Birmingham throughout history".
The entire collection, which is being sold by Broderick’s children, is expected to fetch around £50,000 in total.
Broderick’s art collection will go under the hammer on March 7.
1. The Guardian statue outside of the Bullring shopping centre in Birmingham.
The Guardian statue outside of the Bullring shopping centre in Birmingham. The six tonne animal, twice the size of a real life bull was created by Laurence Broderick and erected in 2003. Photo: Jason Wells
2. Bull sculpture sketches
The drawings, by renowned artist Laurence Broderick, were discovered almost two years after his death aged 88. They were stashed under a bed at his family’s home in Cambridgeshire. Photo: SWNS
3. Bull sculpture sketches
Broderick was an internationally acclaimed sculptor specialising in figurative carvings in stone and castings in bronze, often with a nature theme. Photo: SWNS
4. Bull sculpture sketches
Sketches of the iconic Bull statue in Birmingham have been found under a bed – and are being auctioned for thousands. Photo: SWNS