A secret affair with the potential to rock the British Government was revealed yesterday, leaving Prime Minister David Cameron "stunned".
The identities of those involved cannot be revealed for legal reasons, but it is said that the relationship was between two middle-aged figures and has now come to an end.
The revelation could have political implications for Cameron, who has been hit by a number of scandals recently.
He was said to be "stunned" by the news, the importance of which he realised "immediately".
In meetings the Prime Minister and his aides discussed the potential implications of the affair becoming public, and the damage it could do to major political plans, the Mail on Sunday reported.
"This revelation is dynamite. None of us could believe it when we first heard it. Then we just thought, 'What a complete mess'," a source told the newspaper.
However, a Downing Street spokesman said yesterday: "There have been no crisis talks or meetings."
It is believed that Cameron was told about the affair - which has also caused those involved personal distress - in recent weeks. No Cabinet member is believed to be involved, but it is said that the affair could be as publicly shocking as that between John Major and Edwina Currie which was kept secret for almost two decades.