"Grand Theft Auto: London, 1961 is a freely distributed mission pack to Grand Theft Auto and Grand Theft Auto: London, 1969 released on 1 June 1999. It is the third game in the series. Like London, 1969, it uses the same game engine as Grand Theft Auto. As the name implies, the game takes place in London in 1961, eight years before the events that took place in London, 1969. The 1961 setting makes this mission pack the earliest Grand Theft Auto game chronologically."
Source: Wikipedia, "Grand Theft Auto: London, 1969", available under the CC-BY-SA License.