The Treaty of London in 1864 was in regards to the United Kingdom ceding the United States of the Ionian Islands to Greece. The United Kingdom had held an amical protectorate over the islands since the 1815 Treaty of Paris.
The federated United States of the Ionian Islands included seven island states lying around the coast of the Peloponnese peninsula. Of the seven, six lay in the Ionian Sea, off Greece's western coast. These six states were Corfù (Kerkyra), Ithaca, Paxò, Cephalonia, Zante (Zakinthos) and Santa Maura (Lefkas). Cerigo (Kythera) was also a state of the federation, although it is situated south of the Peloponnese.
Ever since Greece had achieved independence in 1832, the people of the Ionian islands had resented foreign rule. At a Cabinet meeting in 1862, British Foreign Secretary Palmerston, decided to cede the islands to Greece. This policy was also favoured by Queen Victoria. The practical reasoning was that maintenance of ownership in the area was too expensive. Besides, the islands did not have great strategic importance; the United Kingdom would still maintain a strategic presence in the Mediterranean from the island of Malta.
After long negotiations with Greece, the Treaty of London was signed by Greek delegate Charilaos Trikoupis on March 29, 1864. On May 2, 1864 the British departed and the Ionian islands became three provinces of the Kingdom of Greece, although Britain retained use of the port on Corfu.
This can be seen as the first example of voluntary decolonization by Britain.