In 1989, 28-year-old Robert Tibbles bought a medicine cabinet artwork full of bottles of pills for £600. His friends derided it as “crap” and told Tibbles he had been ripped off and should return it.

On Thursday that medicine cabinet, called Bodies by Damien Hirst, is expected to sell for between £1.2m and £1.8m in an auction of Tibbles’ entire Cool Britannia collection, which also includes works by Michael Craig-Martin and Gilbert & George.

“I always thought the medicine was a very clever piece, and it really resonated with me,” Tibbles said. “I can’t explain why I had to buy it, but I did. For a long time people were really quite rude about it, essentially they were saying ‘what is that doing in your drawing room?”.

Then a young City trader, he bought the cabinet shortly after it was shown in Hirst’s degree show at Goldsmiths, making Tibbles the artist’s first customer alongside Charles Saatchi, who bought two of the original four cabinets.

Hirst described Tibbles as a “lovely man”, whom he thanks every day for helping him get started in the art market.

“I remember Robert very well, he was so excited by the art,” the artist said. “It was in the days when I installed my own work in people’s houses, so I went round and met him and he made me tea,” Hirst said.

“Robert is a proper collector and I’m really touched he kept and lived with my work for all those years.”

“He believed in me when no one did,” Hirst said in an Instagram post. “Not many people bought my work in those days and the ones that did sold it pretty quickly … The medicine cabinet is one of the first 12 I made in 1989.

“It was before I had assistants and so I made it myself in my kitchen, buying all the bits from B&Q, and I collected the medicine bottles and boxes from my local hospital in Greenwich.”

Tibbles, who started buying Hirst’s work with his annual bonuses from the US investment bank PaineWebber and later the Swiss bank UBS, said he shed “a few tears” when he read Hirst’s Instagram post.

As well as Bodies, Tibbles also bought an early Hirst spot painting, Antipyrylazo III (1994), which is also up for auction. It carries a guide price of £900,000-£1.2m and is Tibbles’ personal favourite.

There is also a butterfly painting called Summer Breeze and a spin artwork – a “beautiful tropical, jungle painting” – both of which are expected to fetch more than £250,000.

Tibbles’ collection of 30 works had been hung in his flat in Earl’s Court, south-west London, which he bought in 1994 to house the collection, because it has large rooms and high ceilings.

The collection is expected to fetch at least £4m when it goes under the hammer at Phillips in Mayfair this week.

Tibbles said hoped to spend some of the proceeds on buying work by young and up-and-coming artists.