Reports of a love affair which could have "significant political implications" for David Cameron if it was made public has set Westminster abuzz.
The Mail on Sunday said it was unable for legal reasons to disclose any details of the relationship, other than the fact that it was now over and that the "middle-aged figures" involved were not serving in the cabinet.
But the Prime Minister was "stunned" when informed about it within recent weeks and "immediately realised the importance", discussing with aides how it could seriously derail his political agenda, the newspaper said.
Number 10 sources insisted there had been "no crisis talks or meetings" as claimed.
The newspaper quoted a senior source describing the affair as "dynamite" - likely to stun the public as much as the 2002 revelation of former Prime Minister John Major's four-year affair with Tory colleague Edwina Currie.
There had also been "great personal distress to innocent parties affected", the source told the newspaper.
Twitter was relatively free of serious speculation regarding the identities of those involved, with many of the social media postings on the topics being jokes about the coalition "love affair" between Mr Cameron and Nick Clegg.
Others posted a doctored picture portraying the Prime Minister in a passionate clinch with US president Barack Obama, Larry the Downing Street cat or suggested Education Secretary Michael Gove was "in love with himself".
Some in Westminster sought to play down the significance of the affair, Liberal Democrat deputy head of media Phil Reilly saying he had been told the names of those involved and was "underwhelmed".
"Was promised Currie/Major level of surprise and something that would impact gov agenda. That's a v high bar. This doesn't meet it," he posted.