Representatives from Labour, the Conservatives and the Lib Dems have questioned the reliability of the election exit poll.
Labour's Harriet Harman said people were still queuing to vote when the exit poll was released.
Michael Gove of the Conservative Party said the exit poll needed to be treated with caution.
The Liberal Democrat's Vince Cable said he found the result "very strange".
David Cameron will fall 19 seats short of a Commons majority, according to the joint BBC/Sky/ITV exit poll.