Francis Bacon was so angry when workmen trampled over the waist-high debris on the floor of his studio that he decided to throw the whole lot out.

But then a friend with whom he had been drinking offered to take the "junk" off his hands.

"It's yours, take what you want," said Bacon, still enjoying what he called his "gilded gutter life" in bohemian Soho.

The friend, a Mr Robertson, scooped up what he could from the floor of the studio in Reece Mews, South Kensington, and took it home in his car.

It was probably the wisest thing he ever did. For the unwanted clutter he has kept in his attic since that day in 1978 could now be worth at least £500,000.

The acclaimed artist's angst-ridden canvases, some of them mutilated, photographs, diaries, letters, signed cheques, old passports and other bits and pieces, will be sold at auction next week.

The extraordinary collection includes three oil portraits, one of which may be of fellow artist Lucian Freud, or Bacon's lover George Dyer.

Auctioneers Ewbank admit they have no idea how much it will fetch but the world record for a Bacon is £7.9million.

Mr Robertson, now 78, said today: "I had no idea that the bits and bobs Bacon was about to throw away might one day be worth a fortune."

The 45 lots include three pictures of dogs, a paintspattered canvas bearing notes in Bacon's hand and four portraits with the faces crudely cut out.

The artist was famous for his drastic self-editing and destroyed a large part of his prolific output. There are four diaries. An entry for 24 October 1971 records: " George [Dyer] died in Paris."

Bacon had met Dyer in 1964, when the artist is alleged to have caught him burgling his flat. Dyer committed suicide in a Paris hotel on the eve of Bacon's first exhibition in the city. Bacon died in 1992.

Auctioneer Chris Ewbank said: " Major works by Bacon sell for millions but there has never been a sale like this, so we don't really know what anything will fetch. Some things may go for hundreds, thousands or even tens of thousands."

• The auction will be held at Ewbank in Send, Surrey, on 24 April at 7pm.