Christopher Edwards said he only found out during the couple’s murder trial at Nottingham Crown Court that dozens, if not hundreds of letters from the actor to his wife Susan were actually fabricated by her.
Edwards made the claim after beginning his defence evidence.
Prosecutors say he and his wife shot dead Patricia and William Wycherley, buried them in their Nottinghamshire garden and raked in £245,000 by pretending they were still alive.
The couple have told the jury the elderly couple were murdered over a bank holiday weekend in May 1998 at their home in Blenheim Close, Forest Town, Mansfield.
After entering the witness box, Edwards was asked by his barrister, Dafydd Enoch QC, about his wife’s extensive interest in celebrities.
The defendant explained that she collected signed photographs and other mementoes, especially relating to the Hollywood legend Gary Cooper.
Edwards also told the jury his wife had told him a story about meeting the football manager Bill Shankly in a London hotel in the 1970s after writing to him.
The 57-year-old then explained how his wife had struck up a correspondence over many years with Depardieu. He said the letters and notes ran into dozens, if not hundreds of items and the written relationship ran from about 1992 or 1993 to around 2005 or 2006.
Edwards said the letters appeared to be genuine because of the way they were written, the use of French constructions and the reference to the actor’s upbringing and other biographical details.
But he told the jury he had recently come to realise they were not from Depardieu.
Asked who he thought wrote all the letters, he said: “I believe it was my wife.”
And asked when he came to this conclusion, he said: “A few days ago, during the course of these proceedings.”
Prosecutors have told the jury the bodies of Mr and Mrs Wycherley, aged 85 and 63, lay undiscovered from 1998 to 2013.
Their neighbours and relatives were told after their deaths that they had gone travelling or had moved to the coast.
Edwards described how he found out about the deaths of his father-in-law and mother-in-law when he and his wife went to their house together.
He told the court Susan Edwards told him: “I can’t stand it any more.”
Edwards said: “Then she said something like, they hadn’t gone away, they’re upstairs.” He said: “She then led me to believe they were upstairs and they were dead.”
The prosecution case is that the couple, who married in 1983, had been in “severe financial difficulties” for much of their relationship and they killed Susan Edwards’s parents for the cash, claiming pension money and selling the Wycherleys’ house. The bodies were found buried in their garden in October last year.
The jury has been told that Susan Edwards, 56, admits the manslaughter of her mother on the basis of provocation. Christopher and Susan Edwards each deny two counts of murder.