European Council President Donald Tusk has said on the 60th anniversary of the bloc's founding treaty that continued unity for the 27 nations remaining following the planned departure of Britain is the only way to ensure the bloc's survival.

Tusk told 27 EU leaders that "Europe as a political entity will either be united, or will not be at all" He spoke during a solemn session in Rome to mark the 1957 signing of the Treaty of Rome.

Tusk said that "only a united Europe can be a sovereign Europe in relation to the rest of the world. Only a sovereign Europe guarantees independence for its nations, guarantees freedom for its citizens."