The Mail on Sunday has published one of the most intriguing of splashes. On a day when the news agenda has been dominated by cash-for-questions newspaper stings (in The Sunday Telegraph and Sunday Times), the Mail's story has tended to pass under the radar.

But the mysterious tale, headlined "No 10 rocked by secret love affair", appears to have the potential to add to the prime minister's problems. It began:

"David Cameron has held crisis talks at Downing Street after being told of allegations of a sensational love affair which has potentially significant political implications for him. For legal reasons, the Mail on Sunday cannot disclose the identities of the people involved or any details of the relationship – even its duration – other than that they are middle-aged figures. The affair has now concluded. But this newspaper can report that when aides told Mr Cameron the identities of the alleged lovers he was 'stunned', and, according to sources, 'immediately realised the importance of the story'."

The story goes on to quote a "senior source" as saying: "This revelation is dynamite. None of us could believe it when we first heard it" and adding that it had caused "great personal distress to innocent parties".

A later sentence states:

"If the affair is revealed, it is likely to cause as much public surprise as the disclosure of the relationship between John Major and Edwina Currie, which was kept secret for nearly two decades until 2002."

Of course, it could be nothing more than journalistic hype, but I somehow doubt that. It's not in the nature of the MoS editor, Geordie Greig.

So what's it all about? What "legal reasons" prevent us from knowing more? Is there an injunction in place, an anonymised injunction possibly?

I have asked and, as yet, no answers. I would guess that the MoS's lawyers have advised those in the know to say no more.

PS: For obvious reasons, to avoid wild and libellous speculation, I am not switching on comments on this posting.