The Declaration entered into force with the exchange of instruments of ratification on May 27, 1985 and was registered by the PRC and UK governments at the United Nations on June 12, 1985. The United Kingdom, most Western governments, and most of the legal experts both in the West and in Hong Kong have always taken the position that the Declaration was a binding international agreement. By constrast, the People's Republic of China and most legal experts in Mainland China have always taken the position that the Declaration is not a binding international agreement. This dispute is significant as it impacts the constitutional theory under which the Basic Law of Hong Kong operates.
In the Joint Declaration, the PRC Government stated that it had decided to resume the exercise of sovereignty over Hong Kong (including Hong Kong Island, Kowloon, and the New Territories) with effect from July 1, 1997, and the UK Government declared that it would restore Hong Kong to the PRC with effect from July 1, 1997. The PRC Government also declared its basic policies regarding Hong Kong in the document.
In accordance with the "One Country, Two Systems" Principle agreed between the UK and the PRC, the socialism system of PRC shall not be practised in the Hong Kong Special Administrative Region (HKSAR), and Hong Kong's previous capitalist system and its way of life shall remain unchanged for a period of 50 years. The Joint Declaration provides that these basic policies shall be stipulated in the Hong Kong Basic Law.
The background of the Sino-British Joint declaration was the expiration of the lease of the New Territories on July 1, 1997. The lease was negotiated between UK and the Emperor of China, and was for 99 years starting from 1898. At the time, the Hong Kong Island was already ceded to UK in perpetuity under the Treaty of Nanjing in 1842 after the First Opium Wars, and the Kowloon Peninsula was also ceded to UK in perpetuity under the Beijing Convention in 1860 after the Second Opium War.
The signing of the Joint Declaration by the Conservative Party government of Margaret Thatcher was a cause of controversy in Britain at the time: some were surprised that the right wing Prime Minister would agree to such an arrangement with the Communist government represented by Deng Xiaoping.
However, some commentaries pointed out that Britian was in an extremely weak negotiating position. Hong Kong is not militarily defensible and receives most of its water and food supply from Guangdong province in mainland China. It was therefore considered economically infeasible to divide Hong Kong, with UK retaining control for Hong Kong Island and Kowloon while returning the New Territories to the PRC in 1997, if no agreements could be reached. As mortgages for property in Hong Kong were typically 15 years, without reaching an agreement to the future of Hong Kong in the early 80's, it was feared that the property market would collapse causing a collapse of the general economy in Hong Kong. Constratins in the land lease in the New Terrorities were also pressing problems at that time. In fact while negotiation concerning the future of Hong Kong started in the late 1970s, the timing of the Declaration was related to land and property factors.