No visa is required for American citizens residing in the United States entering the United Kingdom for purposes of business or tourism for visits of 180 days or less. American citizens residing in the United States do not require a United Kingdom airport transit visa for a visit of two days or less, but a transit visa may be necessary if the American citizen passes through another country on his way to the United Kingdom, says VisaHQ.
Americans wishing to stay in the United Kingdom longer than 180 days should contact the nearest American consulate or embassy for assistance, states World Embassy Page.
Americans for whom the 180-day travel limit is adequate must possess a United States passport that does not expire for at least 90 days beyond the planned return date, although twice that time is advisable. This passport must contain adequate space in the Visas section (a minimum of two unstamped pages) for international transit. If adequate space is not available, it is possible to add more pages in as little as 24 hours, says VisaHQ.
United States citizens needing assistance with passport renewal, an initial application or other expedited services can visit VisaHQ online. For information on embassies worldwide, visit World Embassy Page online.