The purpose of the test, which will initially involve less than 100 selected households, is to determine the market and technical feasibility of offering the service nationwide.
WebTV, which Microsoft acquired last year for $425m (£262.35m), currently has 300,000 customers in the US, up from 250,000 at the end of last year.
WebTV was also launched in Japan last Autumn but subscription figures for the Japanese market were not available.
The service allows customers to browse the Internet and send and receive email using a standard television set equipped with a special remote control and set-top box. Users can roam the Internet using part of their television screen while watching a show, using WebTV's "picture-in-picture" technology.
The British trial is being done in conjunction with British Telecommunications' BT Internet service, with set-top boxes provided by Pace Micro Technology and Philips Consumer Electronics. After the initial limited trial with friends and family members of Microsoft and BT employees, the test will be expanded to several hundred customers throughout Britain.