On 5 July, a panel of the most senior judges in the French appeal system will announce whether Jacques Walter is owed pounds 50m by the French government as compensation for its ban on the export of a Van Gogh landscape he once owned.

A lot more than pounds 50m hangs on the judges' decision, however, including echoes of a 30-year-old murder mystery and a struggle over the ownership of paintings in one of the world's most famous art galleries, with works by Renoir, Cezanne and Matisse.

The Jardin a Auvers is one of the last landscapes Van Gogh painted before his suicide in 1890. Jacques Walter battled for 10 years to be allowed to sell his painting outside the country, but finally ran out of money. In 1992, he popped it into a Paris auction to pay off his debts. Because of the export ban - the government had declared the painting an unexportable 'historic monument' - bidding was restricted to French citizens and the painting fetched only pounds 5.5m.

Jacques demanded compensation from the state, and earlier this year a Paris court awarded him pounds 50m. The Van Gogh would have been worth pounds 32m on the international market in 1992, according to the judge, and on top of that, Jacques was entitled to be compensated for loss of interest on capital. The government immediately appealed, and after hearing arguments from both sides earlier this month, the appeal judges are now pondering their verdict.

Jacques, now 86, has lost interest in litigation and handed the Van Gogh battle over to his son, Jean-Jacques, a sprightly 62-year-old. A tall, grey-haired man, shy but precise, he lives in a spacious apartment overlooking the Luxembourg Gardens. 'We still have something left from the pounds 5.5m to spend on lawyers,' he told me as we relaxed on his orange suede chairs.

The Walters are quite a family. Their fortunes were founded by Jean Walter, Jacques's father, an architect who, thanks to Moroccan mining rights handed over by a client unable to pay a bill, became one of the richest men in France. He put Jacques in charge of developing the mine, which turned out to be one of the richest sources of zinc and lead in the world. By 1955, Jacques had made enough to splurge on a Van Gogh. He bought the Jardin a Auvers from the Knoedler Gallery in New York for the equivalent of pounds 160,000 and shipped it back to his Paris apartment as a 'temporary import'.

Meanwhile, his wealthy father was building up a considerable art collection, including a handful of great Cezannes and a Gauguin. It was sold to the state by his widow, Domenica Walter, between 1958 and 1963 and is now housed in the Orangerie Museum in the Tuileries Gardens in Paris.

Domenica was a great beauty - this can be seen from the portraits of her by Derain and Marie Laurencin in the Orangerie. Jean Walter was her second husband. Her first was Paul Guillaume, one of the most brilliant avant-garde art dealers of the inter-war years. Guillaume discovered Modigliani and Soutine, was the first serious collector of African art, and helped Dr Albert Barnes form his famous collection in Philadelphia - now touring the world to great acclaim. When Guillaume died in 1934 at the age of 43, he was planning a private museum, which, according to an article written shortly before his death, would be 'the most brilliant representation of the great French paintings of the last 50 years, second only to that of the Barnes Foundation'.

But Guillaume appeared to have died without leaving a will, or at least Domenica couldn't find one. Since he had no close relations and he and Domenica had no children, his property, under French law, should have passed to the state. The situation was desperate. So Domenica stuffed a pillow up her front, according to the family, and announced to the world that she was pregnant. Then she found a baby to adopt and retained Guillaume's fortune in his name. Ten months later the will turned up, leaving her everything, but by then the deed was done. Little Jean-Paul Guillaume, as the adopted baby was called, would present no problems until he was 21 when he would, by rights, come into his inheritance. Domenica did not stay single for long. In 1941, she married one of her former husband's clients, none other than Jean Walter.

Despite the fact that her new husband was by now fabulously wealthy from his mine, buildings and other business interests, Domenica was still unwilling to see little Jean-Paul Guillaume make off with a slice of the Guillaume fortune. She is alleged to have hired a former officer in the Resistance to murder her 'son'. But the plot backfired as her chosen executioner went straight to the police. Domenica was arrested, but when the case came to court, the evidence was considered insufficient to prove that she had intended murder. Jean-Paul survived, and is still alive, but Jean-Jacques Walter does not know of his whereabouts.

Shortly after Domenica's trial, the Guillaume and Walter art collections were sold to the state for 15 per cent of their current market value - and Domenica gave the Friends of the Louvre the money to pay for them. The press claimed her gesture was a pay-off for the not-guilty verdict. Domenica handed over 144 paintings, including 24 works by Renoir, 16 by Cezanne and 10 by Matisse. Some came from Guillaume, but it is not clear which of the others had been bought by Jean Walter and which by Domenica herself - albeit with his money.

To this day, however, the Culture Ministry cannot bring itself to name the benefactress. On the museum posters it is described as the Jean Walter and Paul Guillaume collection. It is now worth pounds 700m, according to Jean-Jacques Walter.

Why did Domenica bother to sell the collection to the state, rather than making a straight donation? Jean-Jacques says: 'She knew that under French law the Walter children from his first marriage could have reclaimed paintings that were donated and she didn't trust them.' French law is strict about the inheritance rights of children; had the art works been sold at their true market price, the children would have had a claim against Domenica. In the circumstances, Jean-Jacques believes, they still have a claim against the state.

Which brings us back to the painting in question. In order to bring pressure on the government to pay the pounds 50m indemnity, Jean-Jacques has gone to court to reclaim the paintings Domenica sold to the state. He says that her deal was 'a donation disguised as a sale' and is enthusiastically seeking to prove the government's complicity in this lie. If he wins, the Walter family intends donating the collection to a 'better behaved' French-speaking country. 'It will go to Geneva, Brussels or Montreal,' he says.

Jean-Jacques's poor opinion of French politicians and civil servants was dramatically underlined earlier this year when he gave evidence to a public inquiry into government corruption. He claimed that his father's lawyer had twice reported to the family that an export licence for the Van Gogh could be obtained by paying baksheesh to the Minister of Culture. He alleged that Jack Lang, the minister from 1981-83, had asked for a pounds 5m payment and his short-lived successor, Franois Leotard, for pounds 3m. Both former ministers have denied the allegations. Jean-Jacques refused to discuss the corruption issue with me. 'I prefer that France washes its dirty linen in private,' he said.

He says that his legal battles are not about money. Jean Walter left nothing to his son, and his two sisters inherited only modest sums. But the children have never sought redress since they were aware of their father's strongly held view that inherited money was 'morally damaging' to the recipient. The bulk of his fortune was divided between scholarships, museums and Domenica. Working the Moroccan mines provided Jacques with a comfortable fortune of his own, while Jean-Jacques has made a pile out of electronic engineering. He subscribes to the family view that inherited money 'causes a child to degenerate'. As he sweeps an arm around the luxurious apartment, he emphasises that it was his electronics business 'that paid for all this'.

Jean-Jacques claims he is fighting to uphold the rights of the private citizen vis-a-vis the state, and to encourage the resurrection of Paris as a world art centre. 'In a modern democracy, the state is drunk with power,' he says, quoting Lord Acton's dictum: 'Power corrupts; absolute power corrupts absolutely.'

He believes that state restrictions on art exports have been responsible for shifting the centre of artistic creativity from Paris to New York in the post-war years. 'In 1920, France accounted for 60 per cent of the world art market; now it has only 10 per cent. And it is not just a matter of economic strength; the UK has 30 per cent of the market though France is richer than the UK.' He is hoping that his litigation will render France's export restrictions unworkable and, eventually, result in the return of artistic activity to Paris.

Many works of art have been classified as 'historic monuments' under a 1913 law to restrict exports. But the Walters are the first owners to demand the indemnity that the law states should be paid. If they succeed, it will result in hundreds of other claims and effectively demolish the government's use of the law.

Jean-Jacques is not optimistic about a favourable verdict on 5 July, however. French courts have a tendency to bow to political pressure. As a form of reinsurance, he has also taken the issue to the European Court of Human Rights. 'According to the treaty, you can't take someone's else's property without paying,' he says. 'I say that banning the export of the Van Gogh without paying is illegal.' The court is expected to issue its opinion in September.

'And if you win on July 5?' I ask. 'The government won't pay the indemnity,' he replies with a laugh. 'Of that, I am certain.' He has reviewed his options. 'As a private citizen, I do not have the right to seize state property in payment of the debt in France. But a judgment passed in one European country is now valid in any other, and I do have the right to seize French property in any other country of the European Union.' There is a twinkle in his eye. But he refuses to divulge where he intends to try this out.