After being restored to life last November, the Harwell Dekatron / WITCH computer, now stored at the UK’s National Museum of Computing at Bletchley Park, has been certified by Guinness as the world’s oldest working digital computer. From the BBC:

Built for the Atomic Energy Research Establishment in Oxfordshire, it replaced work done by adding machines.

In November, it was rebooted, after a three-year restoration at the museum.

Design and construction of the 2m (6ft 6in) high and 6m (19ft 8in) wide machine known as the Witch, began in 1949.

The 2.5 tonne computer took up to 10 seconds to multiply two numbers but was reliable and often ran for 80 hours a week.
Kevin Murrell with the Guinness certificate Kevin Murrell: “Delighted” with Guinness World Records’ recognition

Kevin Murrell, one of the museum’s trustees, said: “This was at a time when computers weren’t really expected to work for more than five or ten minutes without breaking.

“Today the fully-functioning computer is proving invaluable in teaching our stream of educational groups about their computing heritage.”