British Prime Minister David Cameron on Wednesday described Elizabeth II as a "rock of stability" in Britain, on the day she surpassed Queen Victoria as the longest-reigning British monarch.Cameron chaired the House of Commons in a special session dedicated to the Queen, who has now exceeded the mark so far held by her great great grandmother.The head of government stressed the scope of services provided by Elizabeth II in the last 63 years, which saw 12 prime ministers from Winston Churchill to David Cameron."Her selfless sense of service and duty has earned admiration not only in Britain, but right across the globe," he added.He said that Elizabeth II is a "rock of stability" in a world of "constant change" and also thanked Philip, the Duke of Edinburgh, the Queen's husband, who joined her in her work.Cameron admitted that his best moments with the Queen were the two times she called him to ask him to form a government (2010 and 2015).