After the Rebellions of 1837 and 1838 in Upper and Lower Canada, the British government sent Lord Durham to study the political situation in the British North American colonies. In his report, Lord Durham recommended, in particular, that the two Canadian provinces be united to form a single province. United Canada was thus born, and consisted of Canada East (formerly Lower Canada and the precursor of modern-day Quebec) and Canada West (formerly Upper Canada and the precursor of modern-day Ontario).
The Act of Union was sanctioned on July 23, 1840, by Queen Victoria and came into effect on February 10, 1841.
The Act of Union was the main reason for the political instability that reigned in United Canada until 1867. Because the Union gave equal representation to both parts of the colony, some members of the political elite, both francophone and anglophone, were calling for rep by pop (representation by population). The situation eventually became intolerable, and led to the Great Coalition in 1864, and ultimately to Confederation in 1867.
Lacoursière, J. ; Provencher, J. ; Vaugeois, D. -- Canada Québec : synthèse historique. -- Montréal : ERPI, 1976. -- P. 311-319.
Monet, Jacques, S. J. -- "Act of Union." -- The 1999 Canadian encyclopedia : world edition. -- Toronto : McClelland & Stewart, 1998.