The Year to Date figure is 14.5 percent down on the first five months of last year.
Alan Nolan, Director General of SIMI (pictured) says the May figures are continuing the trend of what has been a very poor year and it's clear that there has been even less spending in the domestic economy this year than in recent years, which were not good to begin with.
Nolan said: "At this time of the year the Motor Industry is usually beginning to ease down, as the second half of the year approaches, and traditionally sales begin to tail off until the New Year. This year, with the new '132' registration beginning on 1st July, there are now real benefits for consumers buying at this time of the year and the Industry is gearing up for some extra interest and activity in July."
He added that June should be a really good shopping month for consumers, with great offers and trade-in values to buy this month, as well as the potential for a new 132 plate.