Referendums in the United Kingdom are occasionally held at a national, regional or local level. Historically, national referendums are rare due to the long-standing principle of parliamentary sovereignty. There is no constitutional requirement to hold a national referendum for any purpose or on any issue; the UK Parliament is free to legislate through an Act of Parliament for a national plebiscite to be held on any question at any time, but these cannot be constitutionally binding on either the Government or Parliament, although they usually have a persuasive political effect.