Dominic Lawson: The only conspiracy over Diana's death was Fayed's bid to manipulate the British public

So there really was a conspiracy surrounding the circumstances of the death of Diana, Princess of Wales. That much has finally been made clear at the conclusion of an inquest which has lasted six months and heard evidence from more than 250 witnesses.

The conspiracy that has been revealed is not, however, the one which has captivated the weirdos of the blogosphere – and many others – for the past decade. As the coroner, Lord Justice Scott-Baker, told the jury, in all those months they had heard "not a scrap of evidence that the Duke of Edinburgh ordered Diana's execution [or] that the Secret Intelligence Service or any other Government agency organised it."

Yet there was a conspiracy to cover up the true circumstances of Diana's death and that of her boyfriend Dodi Fayed – a tragic accident involving an intoxicated and over-excited driver (an employee of Mohamed Fayed's Paris Ritz) hotly pursued by a cavalcade of adrenalin-fuelled paparazzi. Instead, Mr Fayed, the billionaire owner of Britain's most vulgar department store, conceived what amounted to a gigantic deception: to persuade the public that Diana's death was no accident – still less the result of any inadequacies on the part of his own employees.

The conspiracy involved framing the Duke of Edinburgh as a man so enraged at the thought that his ex-daughter-in law would marry a Muslim that he would have both of them murdered. In time, the conspiracy of alleged agents of the Duke would spread to include (in no particular order) Tony Blair, the late Robin Cook, Diana's own sister, Sarah, and the entire French medical team on the night of the accident.

Aided by his superficially respectable spin-doctor, Michael Cole, Mr Fayed had remarkable success in persuading elements of the tabloid press, notably the Daily Express, to give the conspiracy a fair wind.

Yet it was all based on lies – as detailed questioning under oath in Lord Scott-Baker's courtroom ruthlessly exposed. The employee given primary responsibility for "discovering" the Royal Family's involvement in a calculating and savage double murder was John Macnamara, a former police superintendent who had become Mr Fayed's "security chief". The hard-faced Macnamara must have made many people "crack" in his time, but it was he who cracked in the courtroom.

After much wriggling, he finally admitted that he had lied to the BBC, among other broadcasters, when he said he "knew" Henri Paul had not been drinking on the night of the crash; in fact he had been in possession of M. Paul's Ritz bar bill on the night in question, which showed that the driver had consumed two Ricards (the alcoholic equivalent of four single whiskys).

To be fair to John Macnamara, he was not the inventor of the main elements in the conspiracy, designed to establish a "motive" for the murder of Diana and Dodi – that they had already agreed to get married and that she was pregnant with his child. The latter claim began to be put by Mr Fayed only in 2001 – via the Daily Express: suddenly he began to insist that Diana had told him this extraordinary news on the night of her death.

As Lord Justice Scott-Baker instructed the jury, with some delicacy: "You will have to decide whether Mohamed Fayed is telling the truth about the pregnancy conversation. If he is, it is strange that he sat on this important information for three-and-a-quarter years. It is also difficult to believe how, if the information that Diana was pregnant was only available in a telephone call for the first time an hour or so before the collision, it could have any relevance to the collision."

Even if this was not a grotesque lie by Mohamed Fayed, I have never been able to understand how anyone but the most malign fantasists could assert that the Royal Family would be motivated to order the murder of a Diana pregnant with a "half-Muslim child". Leave aside all issues of common decency and plausibility – although these do matter – why should anyone in the Royal Family even have had murderous thoughts at such a prospect?

Diana had already been cut adrift from the House of Windsor – she had been dramatically stripped of her royal title. The Royal Family, in so far as it can even have a collective thought, might well have been delighted at the prospect of the troublesome Diana leaving Britain for good to live in Beverly Hills with Dodi Fayed – the future scenario outlined by Mr Fayed Senior.

It is, in fact, just another fantasy by Mohamed Fayed that the Royal Family wanted nothing to do with him, or even despised him. Long after successive British Home Secretaries had denied Mr Fayed a British passport (on the understandable grounds of "bad character"), the House of Windsor had been happy to consort with the Harrods owner.

Prince Philip had wined and dined with him, picking up the odd cheque for his pet project, the Duke of Edinburgh Awards Scheme; for year after year, the Queen had Mr Fayed sitting next to her at the Royal Windsor Horse Show, in return for his generous sponsorship.

That sponsorship ended only – at the urging of the Royal Family's own advisers – some time after Diana's death, and it was only in 2000 that the Duke of Edinburgh removed his Royal Warrant from Harrods. In both cases, one wonders why Mr Fayed was prepared to continue his commercial relationships with a family he thought had ordered the murder of his son. Business is business, I know, but this is ridiculous.

The truth is that, while masquerading as the voice of the ordinary people of Britain against the privileged royal establishment, Mr Fayed is completely obsessed with social status. This facet of his character emerged on the day he took the witness stand, when he ridiculed the idea that the Princess of Wales would ever have married her long-term lover, Dr Hasnat Khan, saying: "How can she marry someone like that, who lives in a council flat and has no money?"

I was in court that day, and I thought I could detect a look of bewilderment on some of the jurors' faces, when Mr Fayed so snobbishly dismissed the suitability of a respectable heart surgeon.

Mohamed Fayed at least promised, under questioning during the inquest, that he would accept the verdict of the inquest jury, whatever it is. It would be good if he sticks to that pledge, made explicitly under oath. Of course, conspiracy theorists in the wider world can continue to proclaim their adherence to the legend that Diana was murdered by "the Establishment".

Perhaps they believe that, in so doing, they are showing themselves not to be gullible, that they are not the sort of people to be duped by anyone, no matter how powerful.

In fact, they are – and always have been – the naïve victims of a conspiracy of lies spun at vast cost by a bereaved billionaire. If they don't realise it now, they never will.