A referendum on whether or not Britain should remain in the European Union is one of the key battlegrounds ahead of the election.

This is where the main parties stand on that thorny question:

Conservatives: A referendum in 2017 following renegotiations

Labour: A referendum only if there is a proposed transfer of more powers from London to Brussels

Liberal Democrats: A referendum to approve any further "significant" transfer of powers from London to Brussels

UKIP: An immediate referendum on if the UK should be in or out

Green Party: Yes to a referendum - but wants to stay in the EU

BNP: An immediate referendum

English Democrat Party: An immediate referendum