In 1975 a worker at the car firm Fiat went to an auction of lost property organised by the Italian national railway in Turin. He paid 45,000 lira (£32, equivalent to about £300 today) for two paintings that caught his eye – one a still life and the other an image of a woman relaxing in her garden.

For almost 40 years, the man – whose name has not been made public – kept the pictures hanging in his kitchen. They accompanied him on his move, post-retirement, to Sicily. At no point until last year, Italian police believe, did he realise what a bargain his purchase had been.

Now it has emerged that the paintings are stolen works by the French artists Paul Gauguin and Pierre Bonnard, and the first – a still life dating from 1869 – has an estimated value of between €10m and €30m (£8m -£25m). The second, entitled La Femme aux Deux Fauteuils (woman with two armchairs), is believed to be worth around €600,000 (£500,000).

Stolen in London in 1970, reportedly from the widower of a daughter of one of the co-founders of Marks & Spencer, they were unveiled on Wednesday to applause at the Italian culture ministry in Rome. Mariano Mossa, commander of the Italian heritage police, admitted that the events leading to the recovery were "decidedly original".

Investigations are under way to help establish to whom the paintings should be given, but the likelihood of any British claim is unclear. The Italian police understand that the works were the property of Terence Kennedy, an American author and socialite who married Mathilda Marks, daughter of the businessman Michael Marks. But both Kennedy and Marks are now dead and Mossa said they had been unable to establish an obvious heir.

According to press reports at the time, the two paintings were stolen by thieves on 6 June 1970 from Kennedy's home in Regent's Park, London. "Three men, posing as a police officer accompanying two burglar alarm engineers, last night stole two paintings together worth £155,000 from a house in Chester Terrace, NW London," reported the Observer the following day. The robbers had been let in by the housekeeper, said the news agency UPI. "They asked her to make them a cup of tea, and when she returned the paintings had been taken from their frames and the men were gone."

The Gauguin, which police said was entitled Fruits sur une Table ou Nature morte au Petit Chien (fruit on a table or still life with small dog), still bears the signs of its theft, with what one officer described as a vertical cut just visible on the surface.

But for almost four decades the paintings' current owner, Mossa said, remained unaware of their colourful past. "The worker, it seems clear, didn't know what they were," Mossa said.

It was last year that the man's son, an architecture student in Sicily, was leafing through a book of artworks and noticed a Gauguin that bore uncanny similarities to the painting that had hung for years in his father's kitchen. It was not the same painting, but had unmistakably been painted in the same style.

Armed with photographs of that painting as well as the Bonnard, the Italian heritage police began working last summer to identify the works and establish how they had come to be where they were. They were soon able to verify their authenticity and, retracing the paintings' steps, they decided that the works in all probability were taken by the thieves by train from Paris to Turin, but were abandoned on board, possibly during border checks.

In Turin, anonymous and unclaimed, they then formed part of the lost property collection that went under the hammer in 1975.

What will happen to the paintings now is unclear. Speaking at the unveiling, Mossa said it would be up to the Italian judicial authorities to decide who had legitimate ownership, adding that, as far as the Italian investigators had been able to ascertain, Kennedy and Marks had left no direct heir.

A police source told the Guardian that they were awaiting information from the Metropolitan police's art and antiques unit in London to help clarify the situation, and that it was still likely that a claim of ownership could come from a member of the extended family.

A statement by Scotland Yard said it had been contacted earlier this year for help in tracing the owners but that it had "not been possible to trace the records of the 1970 theft". "The unit was able to establish that the paintings had been sold by Sotheby's in the US in 1962 and advised the Italian authorities accordingly," it added.

As for the ex-Fiat employee, Mossa said he hoped very much that the paintings would be returned to the former Fiat employee.

• This article was amended to alter the caption to reflect that the person pictured is an Italian minister and not a policeman