France, Greece, Italy, Portugal and Spain have regularised more than 3m illegal residents in the past 20 years.

ECONOMIST: Illegal immigrants

One way for both organisations to be sure that each has the other's interests in mind is for conversations between them to be regularised.

ECONOMIST: In defence of Europe

Britain has had a reserve of part-time or retired soldiers - often known as yeomanry - since the Middle Ages but the system was only regularised in 1907 with the passing of legislation creating a Territorial Force.

BBC: Philip Hammond with soldiers