Previously unseen portraits by Francis Bacon have come to light among a collection of his possessions that an electrician saved from being thrown away in a rubbish skip 30 years ago.

Mac Robertson, now 75, says he had been drinking with Bacon in a pub when they discovered that workmen had trampled over the debris littering the floor of the artist’s notoriously chaotic studio.

The volatile Bacon was so angry, Mr Roberston says, that he decided to throw the lot out and was preparing to dump the “junk” into a skip. Mr Robertson says that he offered to take it off his hands and was told: “It’s yours – take what you want.”

The items included several portraits, some of them mutilated, as well as unpublished photographs, diaries and letters. Mr Robertson scooped the lot into three bin-bags and took them home, where he stored them in his attic. They are to be sold at auction next month.

The angst-ridden pictures of the 20th century’s most highly acclaimed British artist now fetch millions. Last month a painting of a pope by Bacon set a record when it was auctioned by Christie’s for £14 million.

Study For Portrait II, an oil on canvas of 1956, broke the previous record of £7.8 million, set in November at Sotheby’s in New York, for Version No. 2 of Lying Figure with Hypodermic Syringe. Only a decade before Bacon painted it, an art charity in Britain struggled to persuade any public collection to accept another major Bacon painting as a gift. Not one of the museums and galleries approached was remotely interested then.

Bacon, a self-taught painter who captured the pain of human existence, is now revered as the greatest British artist since Turner. He is best known for images of screaming popes in which Portrait of Pope Innocent X was converted into a nightmarish depiction of hysterical terror.

In Study for Portrait II, measuring 60in by 45in, Bacon portrayed a bleak embodiment of dejection, conveying on canvas the melancholy brought on by the end of his passionate affair with Peter Lacy. Lacy, who died in 1962, was a troubled man given to bouts of sadistic violence and long periods of self-hatred but he was the first man with whom Bacon fell “head-over-heels” in love.

This collection includes Lacy’s national registration identity card, and previously unseen photographs of Bacon on a terrace with his lover at a pavement café and the top of the Eiffel Tower.

There are also references to another lover, George Dyer. An oil portrait may be of his fellow artist Lucian Freud or Dyer. Two others, one dark, the other ghostly, appear less complete.

Four of his pocket diaries, covering the years between 1966 and 1971, are also to be sold. One entry, under October 24, 1971, records Dyer’s suicide: “George died in Paris.” A later entry reads: “George buried City of London cemetery, Ilford.”

There are also four mutilated portraits. The faces have been crudely cut out by the artist, who was famous for his drastic self-editing and who destroyed a large part of his prolific output.

Mr Robertson said yesterday: “I was in the right place at the right time. I had no idea that the bits and bobs Bacon was about to throw away might one day be worth a fortune.”

Chris Ewbank, an auctioneer of Send, Surrey, said: “Major works by Bacon sell for millions but there has never been a sale like this before so we don’t really know what anything will fetch. It’s unique.

“Some things may go for hundreds, thousands or even tens of thousands. We have no way of knowing.”

The 45-lot sale will be held on April 24, in the week of the 15th anniversary of Bacon’s death.

Throwaway art

— A paper table cloth, torn by Picasso into the shape of a dove, fetched £7,150, ten times its estimate in 1990

— Marcel Duchamp, in love with the wife of the Brazilian Ambassor in Paris, scribbled a heart for “Maria” on a hand-wipe in a restaurant. It sold last year for €125,000 (£84,000)

— Damien Hirst: fixtures and fittings from his restaurant, Pharmacy, sold for £11.1 million at Sotheby’s in 2004. Six ashtrays made £1,600, against an estimated £150. Salt and pepper shakers went for £1,920 (estimate £30-40)

— A portrait by Graham Sutherland commissioned by Parliament to celebrate Churchill’s 80th year was condemned by the statesman. He saw it as a man in decline. His wife, Clementine, is said to have burnt it

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