Reunification the facts:

Who gets to decide whether and when to hold a border poll?

The UK Government has the power to call a referendum in Northern Ireland.

The Good Friday Agreement states that "the Secretary of State" should call a referendum "‘if at any time it appears likely to him that a majority of those voting would express a wish that Northern Ireland should cease to be part of the United Kingdom and form part of a united Ireland."

The Secretary of State would do this by laying before Parliament an Order in Council, specifying the details of the poll and the date on which it will be held.

In the Republic of Ireland, reunification would require a constitutional amendment. This would require legislation to be passed by both chambers of Parliament, before it was put to the people in a referendum.

In the event that either part of Ireland voted against reunification, another poll cannot be held within seven years.