Built between 1961 and 1966 to improve links between England and Wales, the Severn Bridge was the first in the world to have an aerodynamically shaped deck, and was hailed as a great engineering advance.
The deck was designed ultra-slim in sections to reduce the impact of the high winds that regularly batter the estuary.
Tony Banks, a heritage minister, said the mile-long bridge is one of 10 post-war bridges to be awarded special status. "These bridges stand as evidence of the technical innovation of post-war bridge building in England," he said.
James Clune, commercial manager of Severn River Crossing, which manages the bridge, said it was unique both from an aesthetic point of view and in terms of its design.
"Of course, you cannot compare the bridge with the Palace of Westminster in the same terms, but as bridges go it is a very pleasing structure," he said.