Data shows a total of 186,265 new cars were registered compared to 203,585 in the same month last year – a decline of -8.5%.
The SMMT says that business purchases helped to offset declines from private and fleet buyers.
Explains SMMT chief Mike Hawes: “We expected demand in the new car market to remain negative in May due to the pull-forward to March – which was an all-time record month – resulting from VED reform.
“Added to this, the general election was always likely to give many pause for thought and affect purchasing patterns in the short term."
Hawes continues: “Although demand has fallen, it’s important to remember that the market remains at a very high level and, with a raft of new models packed with the latest low emission and connected technology coming to market this summer we expect the market to remain strong over the year."