Her action would force parties to work towards a coalition government rather than having to go to the polls again for a decisive result.
It raises the possibility that Gordon Brown could remain as Prime Minister of a minority government.
Mr Brown would stay in Number 10 until he resigns, Whitehall sources say.
And officials who have held detailed discussions with the Queen's private secretary, Christopher Geidt, say it would be up to Mr Brown to decide when to go.
Even if the Tories are the largest party, but fail to secure a Commons majority, Mr Brown would have the right to make the first attempt to soldier on by coming to an arrangement with smaller parties.
The Queen's potential involvement-which is allowed under ancient conventions, has emerged in behind-the-scenes preparations in Whitehall.