Trident missiles are provided to the United Kingdom under the terms of the 1963 Polaris Sales Agreement which was modified in 1982 for Trident. British Prime Minister Margaret Thatcher wrote to President Carter on 10 July 1980, to request that he approve supply of Trident I missiles. However, in 1982 Thatcher wrote to President Reagan to request the United Kingdom be allowed to procure the Trident II system, the procurement of which had been accelerated by the US Navy. This was agreed upon in March 1982. Under the agreement, the United Kingdom paid an additional 5% of their total procurement cost of $2.5 billion to the US government as a research and development contribution.