The British Government sets the bar low when it comes to passport requirements for U.S. citizens who are visiting on vacation. A passport that's valid for the duration of their stay is all that's required. Stricter rules may apply if you are visiting the U.K. for other reasons, however, like passing through on the way to Europe or for work.

Visiting the U.K. as a Tourist

The United Kingdom is famous for its history, culture, stately homes, public gardens and many well-known sights and events, which make it worth returning to again and again. For U.S. citizens visiting the U.K., entering the country is straightforward. American tourists don't need a visa to visit the U.K. The only rule is that the passport must not expire before their planned departure date. No passport checks take place at the borders of Scotland or Wales or when entering Northern Ireland from the mainland U.K.

Visiting the U.K. and Europe

Though the Atlantic Ocean is affectionately called "The Pond," crossing it is quite a journey. Consequently, many visitors from the U.S. like to combine a U.K. vacation with visits to nearby France, the Netherlands or other European countries. Other visitors are only transiting in the U.K. on their way to a European destination. To enter one of the 26 European countries that's covered by the Schengen rules (the EU's free movement agreement), U.S. citizens' passports must be valid for six months after entry.

Academic, Work or Business Visit

Detailed rules apply to passport requirements when visiting the U.K. for academic reasons, or for business or work. Visitors who are staying for less than six months don't require a visa but they must have a passport that's valid for six months on arrival and contains one blank page.

When staying less than six months to perform work as an academic, legal, artistic or sports expert, be prepared to show immigration officials an invitation from the paying authority, accommodation details and proof of expertise. Business visitors attending a conference, meeting or training, and other visitors taking part in sports-related activities, performing as an artist, accompanying students or undertaking research also don't require a visa, but they must be able to show evidence they can support themselves during their visit.

To work in the U.K. for less than six months, apply online at the Access UK website for a temporary work visa that relates to the area of work. Sports, arts and entertainment, skilled work, domestic labor, volunteer work and religious and charity work are some areas where temporary work visas are available.

Staying Longer Than Six Months

To stay in the U.K. for longer than six months, apply for the appropriate visa online at the Gov.UK website for applications in the U.S.A, ideally around three months before the planned travel date. A valid passport is one of the required documents for any visa application, and the other required documents relate to the reason for the application. For example, temporary charity workers must include a certificate of sponsorship that was awarded by a licensed U.K. employer. Other reasons for applying for a visa to remain in the U.K. for longer than six months include the performance of skilled or volunteer work, or to work in the country as an investor or entrepreneur. After applying online, applicants visit U.S. Department of Homeland Security Application Support Center to have their photograph and fingerprints taken, at which point, they mail their stamped biometric receipt, supporting documents, application form and passport to UK Visas and Immigration at the British Consulate General in New York.