Longest measured comet tail

The longest comet tail ever recorded was that of the comet Hyakutake which measured 570 million kilometres 350 million miles long - more than three times the distance from the Earth to the sun. This long tail was discovered by Geraint Jones of Imperial College London on 13 Sep 1999, using data gathered by the ESA/NASA spacecraft Ulysses on a chance encounter with the comet on 1 May 1996.
This is only the fourth comet tail encountered by a spacecraft and the one Farthest from the sun. Hyakutake was discovered in 1996 and named for the Japanese amateur astronomer who located it.
Although encountered in 1996, it was only reported as a comet tail after two instrument teams from Imperial College, London, UK, and ESA's Ulysses Project Scientist went through old Ulysses data and made the discovery independently of one another. The results were reported in Nature magazine in April 2000.

Although other comets undoubtedly have longer tails, this is the longest ever measured.