BBC Scotland's investigations into the tax arrangements of a company at the time it was owned by Irish billionaire Dermot Desmond were sparked by the leaked documents known as the Paradise Papers.
It appeared that Mr Desmond, the largest shareholder in Celtic FC, had owned an exclusive private jet company that used an offshore tax haven to avoid taxes.
The BBC wrote to Mr Desmond five times but he failed to respond to any written inquiries.
So BBC Scotland's Mark Daly approached him outside Celtic Park on Tuesday last week, ahead of the Celtic v Bayern Munich Champions League match.
Mr Desmond said it was "absolutely wrong" to suggest the Isle of Man Execujet company was a tax avoidance vehicle.
The next day he sent a letter to the BBC:
Here is the full text:
Dear Mr Daly,
Are you a Rangers supporter?
You ambushed me last night on the way to a Celtic game in the company of my family and friends. This was unwarranted and showed no respect for my privacy.
I have not previously responded to your ill-informed questions. However, as I informed you last night, during my period of majority ownership, Execujet was tax and regulatory compliant in all jurisdictions it operated in; nor was it under any investigation. There are good operational reasons for having a presence in the Isle of Man and many international companies do so. Similarly, my aircraft arrangements and my shareholding in Rietumu are fully tax and regulatory compliant. Any allegation to the contrary is a lie.
It is part of due diligence criteria for all my investments that they are tax and regulatory compliant.
You are trying to create something that does not exist.
If you choose to publicly state that my investments are not tax and regulatory compliant or make any other untrue allegation about me whether by innuendo or otherwise, I will take action against the BBC and against you personally in both Scotland and Ireland for defamation and breach of privacy.
If you broadcast the video footage taken by your camera crew last night, I request that you broadcast it in full along with the entire contents of this letter. I also put you on notice not to delete or edit any of the video footage, as this will be required for court purposes.
Dermot F Desmond
The papers are a huge batch of leaked documents mostly from offshore law firm Appleby, along with corporate registries in 19 tax jurisdictions, which reveal the financial dealings of politicians, celebrities, corporate giants and business leaders.
The 13.4 million records were passed to German newspaper Süddeutsche Zeitung and then shared with the International Consortium of Investigative Journalists (ICIJ). Panorama has led research for the BBC as part of a global investigation involving nearly 100 other media organisations, including the Guardian, in 67 countries. The BBC does not know the identity of the source.
Paradise Papers: Full coverage; follow reaction on Twitter using #ParadisePapers; in the BBC News app, follow the tag "Paradise Papers"
Watch Panorama on the BBC iPlayer (UK viewers only)