What is a hung parliament? And what happens next?
The 2015 General Election is the most closely fought in decades and final rallying appeals by party leaders did nothing to break the opinion poll deadlock.
A majority government needs 326 seats in the House of Commons and if no party appears a clear winner on Friday morning, Britain will be facing a hung parliament.
If this is the case, the incumbent Prime Minister, David Cameron, will remain in office and living at Downing Street until a new Government is decided.
An incumbent Government has until the meeting of the new Parliament on 18th May to see if it can put together a deal to stay in power.
It must command the confidence of the House of Commons - with votes on the Queen’s Speech, on 27th May, traditionally regarded as motions of confidence.
If a Government is defeated on a motion of confidence, the Prime Minister is expected to tender the Government’s resignation immediately.
Ed Miliband could also argue that the incumbent Government should resign before 27th May, if he feels that there is strong support for Labour and the PM's Queen's Speech will be rejected in the Commons.
Options for a new Government in the event of a hung parliament:
Britain may be left with a majority coalition, such as the Conservative-Lib Dem coalition of 2010
A new Prime Minister may take up a looser "confidence and supply" deal with allies
A governing party could form a minority coalition with a smaller party - this leaves them without a majority and at the mercy of Commons support for every vote
A party without a majority could attempt to form a minority government, which leaves them reliant on vote-by-vote support
What happens when an election does not result in a clear majority:
The incumbent government remains in office unless and until the Prime Minister tenders his and the government’s resignation to the Monarch.
It is entitled to await the meeting of a new Parliament to see whether it can command the confidence of the House of Commons.
If it can't, it is expected to resign immediately.
Where a range of different administrations could potentially be formed, the expectation is that discussions will take place between political parties on who should form the next government.
The Queen would not expect to become involved in such discussions but the Palace would be kept informed of progress.
The Queen's role:
The appointment of a Prime Minister is the prerogative of the Sovereign after a General Election.
The role is normally secured by appointing the leader of the party with an overall majority of seats in the Commons but in exceptional circumstances The Queen might need to exercise discretion to ensure that her Government is carried on.
When a potential PM is called to Buckingham Palace, The Queen asks him or her whether they will form a government and the usual response is acceptance.
Where the situation is uncertain, as it was with Sir Alec Douglas-Home in 1963, a potential Prime Minister can accept an exploratory commission, returning later to report either failure or success.
After a new Prime Minister has been appointed, the Court Circular will record that "the Prime Minister Kissed Hands on Appointment" but this is not literally the case. In fact, the actual kissing of hands will take place later, in Council.
More: General election 2015: What to expect when the polls close