Liberal Democrat leader Nick Clegg has held private meetings with Gordon Brown and David Cameron as talks continue over forming the next UK government.
It is understood that Mr Cameron was unaware that Mr Clegg had also met the Labour leader for talks on Monday.
Earlier the official Tory and Lib Dem negotiating teams had reported good "progress" as negotiations continue.
The Conservatives won the most MPs and the most votes in the election - but failed to win an outright majority.
They have been talking to the Lib Dems about terms for working together in government since Friday.
But it has now emerged that the Lib Dem negotiating team has also been in what appear to be parallel talks with Labour - meeting Lord Mandelson, Ed Miliband, Ed Balls and Lord Adonis in secret at the weekend.